Front Derailleurs have a ton of variations to consider when finding one for your bike.
Speed
If you're using a derailleur, it's for a 2x or 3x setup. The inner plate of a triple is larger to reach a smaller inner ring.
Attachment
The most basic attachment is a clamp, but there can be small, medium, or large sizes for various seatpost diameters. You can use a shim if you have one that is a size too big.
You may often see braze on, which has a tab on the frame to attach to. There are clamp adapters if you have the derailleur but no attachment point on the bike.
Mountain bikes have direct mount, which is similar to braze on, or can be attached via a bottom bracket adapter. There are also direct mount clamp adapters.
Cable pull
Depending on the cable routing, there are top or bottom pull options.
Chainstay angle
Derailleurs are made for a given angle between the chainstay and seatpost. If this angle is off, the derailleur won't be correctly aligned for the chain. Some models come in multiple options.
Common options are 61-66°, 63-66° or 66-69°.