Each potentiometer is metered and matched in sets of pairs and quads (when possible). They are then categorized by their DC Resistance reading. These values can have a large impact on your overall tone and it's important to find the best option for your sound and setup.
The higher the value the brighter your wide open tone will be. The lower the value the darker/warmer (depending on perspective) your wide open tone will be. One player's sparkling highs is another player's shrill ice pick highs. One player's warmth is another's muddy mess.
Picking potentiometer values can be daunting and the best place to start is with your current setup. Measure the potentiometers that are in your guitar now - give yourself a starting point so that you know which direction you want to go.