LDO LP2992 5V Naming
If you look at the LP2992 datasheet, you'll see that the "A" versions have tighter tolerance on their specifications.

The "I" seems to denote industrial temperature range, although they never explicitly say this. They don't seem to offer any other temperature range anyway.

"M5" denotes the SOT-23 package.

The "NOPB" notation means "no lead" (RoHS-compliant).

However, I can't figure out what the "X" denotes. I suspect it has something to do with how the part is delivered on different kinds of reels for high-volume manufacturing.