Most of our bass pickups are available individually with matched "Bass" and "Treble" voicing.
Usually, the Bass pickup (-B) is installed in the neck position, and the Treble pickup (-T) is installed in the bridge position. We realize that -B can be confused to mean bridge. Terms like 'front' and 'back' or 'rear' can be equally confusing.
The bridge pickup (-T) generally has a little higher output than the neck pickup (-B). This helps balance for the differences in string traverse for the different positions and allows the level to be a little higher when the soloed at the bridge. Because the string is fixed at the bridge, the string cannot move as far much the bridge. Realizing that the pickup detects a change in position of the string, you can see that the reduced movement will result in a reduced output level. At the neck position, the string can move more freely, so it can create a higher output level in the pickup.
That said, there are lots of builders who like to reverse them on purpose. The intent being to maximize the tonal differences and output levels between the two pickups in the two positions. That is why we refer to them as –B and –T instead of specifically labeling them as neck and bridge or front and back. That abstraction gives builders the flexibility to install them how they like without instrument owner thinking it is “wrong”.
Jazz bass pickups are available individually or as matched pairs. For Fender shape Jazz bass pickups, the bridge pickup is usually longer than the neck pickup, so we label them as -L (long) and -S (short). There are cases where the short size is used at the bridge, such as Mexican made Jazz basses. In this case, you can us an -SB (short bridge) pickup in the bridge position to get the same voicing as the equivalent -L version. Similarly, there are a number of builders who use the long size at the neck, so it this case you would use a -LN (long neck) to get the same voicing as the equivalent -S version.
P-bass pickups are currently labeled as -B and -T. The -B pickup is for the bass strings - E&A for 4 string, and B,E&A for 5 string. The -T is for the treble strings - D&G for both 4 string and 5 string. In the past, these have been labeled in other ways like -N and -S for instance. In this case, realize that the treble pickup always has 2 cables attached - the one that goes to the control cavity, and the one that goes to the bass pickup. We wire them this way because the treble pickup is closer to the control cavity than the bass pickup. If they are reversed on 5 string instruments, you will not pick up the B string.