A2 - warm, rounded high end, lots of mids, not much treble, loose low end, lots of vintage dynamics, low output.
A3 - like an A2 but with more treble and less bass.
A4 - flat EQ, medium output. no big EQ push, which lets the PU's and wood's tones come thru.
A5 - bright, sharp high end, scooped mids, lots of bass. firm low end, relatively high output.
UOA5 - unoriented A5 - will sound like the middle ground between an A2 and A5, warm but with a little more treble than an A2, lots of dynamics, medium output.
A8 - warm, lots of mids, some treble but not an excess, firm low end, high output.
All A2's, A3's, and A4's are unoriented, along with UOA5's. A5's and A8's are oriented, which means the inner 'grain's in the metal have been magnitized and aligned in one direction when the metal was being formed, which makes a stronger magnet and a more uniform magnetic field. Unoriented magnets have random 'grain' and a more complex and varied magnetic field.
Polished magnets have a sharper high end. Roughcast magnets have a slightly smoother high end. Not a huge difference.
Alnico mags are numbered by output, more or less, so A2's and A3's are low output, A8's are high.
Ranked in order of warmth: RC A2, A2, UOA5, A8, RC A4, A4, RC A3, A3, RC A5, A5.
So if you know what magent is in your HB or P-90, and you want to change the EQ, choose a magnet that will move it in the direction you want.
Some common swaps are:
JB - can be bright and harsh. Has an A5. An A2 adds mids and takes away some treble.
'59N - can have a boomy bass in mahogany. Has an A5. An A4 or A3 takes away some bass.
'59B - bright and scooped. Has an A5. An UOA5 or A8 fixes both issues.
490R, '57 Classic, PGN - can be dark in the neck slot. Have A2's. An A5, A4, A3, or UOA5 add treble.
498T and Custom 5 - bright and midscooped. Have A5's. An A8 or UOA5 add mids and dials down the treble.
490T, PGB, '57 Classic - bridge PU's that can have an overly rounded high end. Have A2's. An UOA5's and A8's add some bite.