I use the custom fields to pass variables to my HomeSeer script that manages my trailers and movies.
Custom 1 is C, H, O, or V depending on whether the movie is in the Changer, on the HDD, and Offline DVD (usually spec feature discs) or VHS (yes, I still have some of these). This variable is used in my HS script to determine which components to power-up/switch to.
Custom 2 is a short string that relates to which trailer to play while the movie is loading. The trailer varies by the movies genre, also, since the length of the trailers vary, this variable is also used to set the length of pauses in the script. examples of strings in this filed are: DD, DDSCIFI, DTS, THX, DDDRAMA, DDCITY, etc.
Custom 3 is a short string that defines the DSP mode my Yamaha receiver will play in. For example SCIFI puts the receiver in the 70mm scifi mode, ADVEN puts it in the 70mm Adventure mode, NORM puts it in the non-DSP mode (straight DD, DPL, DTS, etc.)
Custom 4 is the title number of the actual movie. It's "1" for 95% of my movies, but - for some (Darn you Dreamworks) like Shrek2, Madagascar, and Shark Tale, it's something like 14. This lets me use the Disc Direct Command to access the actual movie, instad of all the other BS they try to make you watch.
Now, that's what I think they are. My problem is I always get Custom2 and Custom3 confused, and can never remember all the strings, so I always have to go to access to see what the proper way to enter them is.