Hello, I have sen that there are different spring ...

Hello, I have sen that there are different spring codes for the front suspension of the 2004 pacifica ex:ZA B, ZNB, ZNA, ZAC, ZNC, etc... Are these different codes for different spring rates?

I own a 2004 pacifica and I would like the suspension to be a little firmer, however I am reluctant to go aftermarket so I was hoping to find  options through OEM.