20 gal sump = 20 gal of water in it?

[Hal]

When you say you have a 20 gallon sump (for example), do you mean that you have 20 gallons of water in it, or do you instead mean that its maximum volume is 20 gallons?

[bowfront]

This usually refers to the maximum volume that the sump can hold. When you run an overflow -> sump -> return pump -> back to tank system the sump must be large enough to accomodate backflow from the tank down into the sump when the return pump is turned off. Normally the sump is somewhere near half full of water when your system is running. Sometimes people will mention the actual estimated volume (while running) that their sump contains but this would most likely be specified in the post.