Let me talk abut how I deal with this.
My first rule of thumb is that I only put things in keywords that are in fact things that an objective observer not at the scene could tell from looking at the picture (assuming he was familiar with the people and activities depicted). So a party at Peter's house would *not* get a keyword of Peter, unless Peter was in the picture. A keyword search for Peter with thus show only pictures of Peter, not pictures that have any indirect connection to Peter. Which is to say, it looks like we are entering are keywords similarly.
So where do I put the info about this in fact being a party at Peter's house? In one or possibly two places. In all cases, this goes into Notes, and from there gets copied to IPTC Headline. That is, the very first thing I do with a cardful of images after import is to select all images shot at a given event and enter into Notes something like "Party @ Peter's". Actually, that's a slight lie: I actually run Batch Set Information with a preset that adds copyright and contact info, and I also use that as an opportunity to set the IPTC location fields. But I'm very non-specific about my locations - assuming Peter, like me, lives anywhere in the general vicinity of Denver/Colorado/USA, that's what gets entered as the location. I could use "Sublocation" to store "Peter's house", but for some reason, that idea doesn't appeal to me, as I think I'd have a hard time being consistent enough about how I did this to be useful, so I don't. If a future version of ACDSee had fields on the db pane for the various components of location (or allowed setting IPTC for multiple files directly), I might be more inclined to do this.
Anyhow, like I said, the information that this is a party at Peter's house goes into Notes and from there, IPTC Headline. If the event is significant enough that I am concerned with being able to find those images again, I will create a Category for it too: Events/Parties/Peter. Or maybe Events/Parties/Peter/Christmas or something more specific like that if Peter is someone who throws a lot of parties. I used to do this for virtually every event, but I've coming to realize that I'd rather save a little time *entering* information even if it means costing me a little time *searching* later, because I already spend far more time entering info than searching. So if takes me a few guesses on how I entered the info into Notes (did I use his first or last name?) in order to find the pictures, that's OK. I am reasonably consistent about how I enter my Notes (always of the form "event @ location"), so I do pretty well in most cases without the need for a category. Eg, "Peter & Party" will show all pictures taken at any party at Peter's, or any party Peter attended. Probably close enough, and then I just make sure the images are sorted by date (the single keystroke "D" does this, because that's how I've customized it) and then it's trivial to scroll until I see the group of pictures from the specific party I want. Again, if I'm concerned when entering metadata that any particular party is so important that I *need* to be able to find it reliably, a Category takes care of that.
Note that this is virtually the same as what the Calender "Events View" can do rather more automatically. But I don't like the fact that this only works if your Events are exactly one day long. It doesn't work well if you've got multiple events on a single day, or if an event spans several days (although I think the latter may work better than the former).
So, now, to later find pictures of Peter himself (not his parties), my first step is usually to do a Quick Search on "Peter". This will of course show me all pictures *of* Peter as well as all pictures taken at his house, or his wedding, or anything else with the word "Peter" in it. 9 times out of 10, I say, "close enough", and just browse the resulting set of thumbs looking for the pictures I want and ignoring the ones he is not in, as it's not worthwhile to me to fine tune the search any more. But if there are just too many false hits and it's frustrating me to have to wade through them, then I can use Auto Categories or the full Search window to find only pictures with a *keyword* of Peter, and then I'll have exactly what I want. If it were possible to customize the Quick Search to search *only* keywords, I'd like this even better.
I think if I had been really clever and anticipated this issue, I'd have reconsidered how I stored the event info in Notes, using some sort of scheme to avoid duplicating keywords. But frankly, it just doesn't seem worth it to me to worry about: without too much effort, I can find what I want as it is.
Before I start to explain what my problem is I have a question about the categories. Can they be written to the files "IPTC" so they are stored in the same way as the keywords? and is there a special field that they should be written to?
As Sam says, yes you can store them, but no, there is no special field for this, and you do lose the hierarchy. I recommend IPTC Supplemental Categories as the most appropriate place, but if I write out my categories and have Events/Parties/Peter/Christmas, all that actually gets written is "Christmas", so it's not really that useful. If you set up a duplicate of your category hierarchy on another machine (or after deleting your own db to start over for whatever reason) and try to read this back in via Batch Set Information, ACDSee will assign the picture to a category called "Christmas", and if Peter's Christmas party is the only one you have, it will get this right. But if you've categories called "Christmas" in several places in your hierarchy, then ACDSee will have guess which to place it in. Some poeple have requested the ability to store (and restore) the whole hierarchy, and perhaps a future version will allow for this. But really, this is just a kludge - no IPTC fields were really *meant* for this. I'm guessing that at some point in the not too distant future, a new standard (probably within XMP) will emerge to provide a reliable place to store this sort of info. And, for that matter, something like a separate "People" field in XMP that was intended specifically to identify the people depicted (separate from keywords).
My understanding so far is that now the categories might come into place. I could assign a category peter to all images of that party.
Would it then be possible to search for the IPTC keyword party in combination with the category peter to see all pictures from that party? Am I going the right track or is this not the way to solve this correctly?
I'd assume you would just want a single category for Peter's party (perhaps Events/Parties/Peter), in which case, no combined search would be needed for this. If you wanted pictures of Peter that were taken at his party, then sure, a search that combined both attributes would make sense. ACDSee's ability to do complex combined searches is limited, but this much is certainly doable.