In general, there are 4 categories of enhancements. In order of least powerful to most powerful, they are: Training (TOs), Dual-Origin (DOs), Single-Origin (SOs), and Inventions, (IOs). You can only slot DOs if at least one of the listed origins on the enhancement matches that of your character, and SOs must exactly match yours. My general advice is to only use whatever drops until around level 22, at which point I typically start slotting level 25 IOs, (remember you can use enh's that are up to +3 above your current level). Unlike regular enh's, you do not outlevel IOs, so those level 25 ones will give you a consistent bonus throughout your character's career. They are also relatively easy/inexpensive to craft and use common salvage, so they are my go-to until I reach the level cap and start working toward set IOs or other specialty enh's. There are more extensive guides out there on how to earn the necessary influence/infamy/information to fully slot yourself up with those aforementioned IOs right away, so check these forums...