Parts are objects with associations, which means that the solid rebuilds itself to a modified state when its generating objects are modified, ie the shape of a profile or path is altered, and actions that have been performed on it, like filleting and shelling, keep the parameters that they've been given, radii and shell thickness. TC activates history associations when the "compound profile" option is checked. ACIS solids are what are commonly called "dumb solids" because their associations are lost, they can only be modified by boolean operations. Dumb solids are more flexible than they used to be because feature recognition means that programs like TC detect what are called "analytic" surfaces, plane, cylindrical and conical sections, and they can be modified through "direct editing" on those faces with tools like facet editor and quick pull, but that breaks the associations too, so better to use those tools on dumb solids rather than using them to make dumb solids! A part can be a composite object, too, it can be modified by editing the generating processes of another object that's been added, subtracted or intersected to it, and the sequence in which operations have been performed can be re-ordered to some extent, because sometimes features depend on other features to be possible. Associations give you a way to redo an operation that's impossibly removed from the undo stack or which was produced at some earlier session, without a complete rebuild. It's a really big subject area, but what it comes down to here is that exploding things in TC dumbs them down, deprecates them a level. That can be useful, if you understand why you're doing it, but I disagree strongly with it being any sort of quick fix for problems, or acceptable common practice. I'm able to model and edit files >100Mb with many complex composite parts in 64-bit TC on a vanilla (slow! 1.44GHz) Atom CPU tablet with 4Mb RAM, and I don't know what the top end file size is.