This is a handy "tool" for scm-coders, that's why I will explain how it works.
First notice that this isn't made by modders, but is added by Rockstar. When you use this, you can choose between more then 1 SCM-file at once. This way you don't have to edit the main.scm in "Program Files > Rockstar Games > GTA San Andreas > Data > Script" everytime, so you can play your own savegames in the original game whenever you want. Like you probably know, you have to start New Game when you make a SCM script, and it is annoying to switch between SCM-files. That is the biggest advantage of MPACKs. Another advantage is that you don't have to start San Andreas everytime you make a change in your code, you can simply start new game.
In an MPACK there are 4 files:
scr.scm (a file in the same format as the normal main.scm)
text.gxt (a normal gxt file)
MPACK.dat (Setting the title for this MPACK)
scr.txt (The source of scr.scm, only needed during development, not when releasing
In this tutorial I will explain how to make a MPACK. First, the MPACK has to be in the "GTA San Andreas User Files" folder, in "My Documents". The first thing you have to do, is make a folder called "MPACK" in User Files folder. You can make up to a maximum of 9 MPACKS, which have there own folder in the MPACK folder. You can compare it to the slots of the savegames. The number behind "MPACK" says which "slot" the MPACK uses.
So in the "MPACKx" folder (x is a number between 1 and 9) have to be 4 files: scr.scm, scr.txt, text.gxt and mpack.dat. The first three will speak for themselves, but the fourth needs some explanation. You have to make a notepad file, in which you have type (for example): "7#Test#". The 7 has to correspond with the "slot" of the MPACK. So when you are using "MPACK7", there has to be a 7. Between the # has to be the name of the MPACK, which you will see in the list ingame. In my case it will be "Test". You have to save this file as "mpack.dat".
Now you are ready to go ingame. Run San Andreas, and choose for "New Game". Here you will find a list of your MPACKs, and you can choose the name of the MPACK you need. As you can see, there is "Test" in slot 7.
Have fun with coding!