It looks like there's a copy/paste error in a help file in my version of CAESAR. In the Dynamic Output Editor, click the PSV icon, and on the "Length of the Vent Stack" entry, I click F1.

"Specifies the length of the vent stack. Add double the lengths of fittings and elbows or calculate the appropriate equivalent lenghts for non-pipe fittings and add the lengths. Typical values for these constants are shown below:"

Then a table showing ratios of gas-specific heats and gas constants for various gases.

Edit to add: Ver: 7.00.01.1600 (Build 141003)

Also, another problem is that if you click F1 on ID of Vent Stack Piping, it instructs you to leave it blank if the ID of the pipe and vent stack match. However, leaving it blank throws up an error, whereas inputting the same value allows it to continue normally.

Thank you Michael, I will have these issues logged for correction.