Define topic attributes and details on filters.
Microsoft recommend that unless you have a good reason to do so, .HxA files should _not_ be compiled into .HxS help files especially if you are integrating content into a VS collection.
1. By not specifying a Title-level .HxA file all topic attributes (defined in the topic headers) will be compiled into the HxS anyway (including any VS attributes).
2. By compiling in a .HxA file (as we used to advise in our tutorials), only those topic attributes defined in the .HxA files are compiled into the HxS help file. The .HxA file acts like a filter to expose selected topic attributes at the HxS title level. So if you declare certain VS (or other) attributes in topic headers BUT forget to declare them in your .HxA you would be in trouble.
Bottom line -- Less error involved in option #1.
3. As always you need a collection-level HxA to expose your attributes at the collection level and to the parent you are plugging into. Note however you do not need to defined the VS attributes in this .HxA since they are already defined in the VS collection you are plugging into.
This info is based on Rob Chandler great knowledge and also available at: http://helpware.net/mshelp2/attribs.htm