The IPL:inst section is used to place (map) models in the game world. You may only place (timed) atomic models and clump models through this section.
You can read in detail about how it works here.
This page does have one tiny oversight though; it does list all available IPL flags, it does not make it clear that the maximum area ID (or interior ID) you can use is 255. This is unique to GTA: San Andreas and this isn't any different on UG-MP.
Furthermore, if you have IPL entries which are rotated on the X or Y axis, you should always set IPL flag 512. A restriction exists in the base game where the flag is ignored unless both X and Y rotations are nonzero, however this restriction is removed on GTA: Underground (and also on UG-MP).
And finally, be careful with LODs! Read our common pitfalls page for more information.
Last but not least, please note that when you use IDE files on DL the model IDs are dynamically allocated for each player. In order to keep things backwards-compatible as much as possible UG-MP uses the ID in both IPLs and IDEs to link models together. It will not be used for anything else but to figure out which item instance uses which model. You therefore have to make sure it is not only unique, but also using a free model ID slot.