Quick question from a newbie. When looking for a good (e.g.) Lot mod on this site, I can search for Lots that have no CC. That's cool.
However, the "Info Panel" in a mod frequently tells me I require multiple game Expansion Packs for that mod.
I have three or four Expansion packs, but I don't intend to buy the whole lot.
Are these mods therefore useless to me?
Thanks!

If the mods are for editing base game functions, or functions regarding the packs you do have, then, in the most part, they would be fine to use. Of course, as you're no doubt aware, if the mods edit functionality for specific packs you don't have, then there would be no point in installing them. Again they probably would not cause any harm, but nor would they do anything in your game.

CC furniture or clothing, on the other hand, may become noticeably problematic if the CC is based on pack content you do not have installed.

If and when a tuning mod that primarily serves base games functions does need an update due to patch changes, - including new data for newer packs, for instance a servo robot cannot get into a swimming pool - it's still fine to use that mod if you do not have Discover Uni Servos in your game. The mod patch changes would be of no effect in your game, only the base game stuff the mod originally alters would still apply.

So then, for lots that require furnishings from packs you do not have installed, one of two things will happen after placing them in the Tray folder. If you are patched up to date, it may allow placing of that lot, but all the stuff you don't have will not be there, and you'll get a screen message saying "missing items." It may replace missing items with base game equivalents. If not patched up to date, it simply will not allow placing of that lot, even if most of the stuff on the lot is from base game (or packs you do own.) Same rule applies to sims and rooms you download regarding missing Maxis content, or CC content based on packs you don't have.

Thank you @coolspear1. That's very helpful!