You need to consult the data sheet. It has to do with the vref which could be internal or external. To some extent it depends on the schematic and the settings. There is a max specified also of course.
The library uses the 6.144v measurement range by default (although the max input should not exceed 5v). And there are single-ended and bipolar options. The chip has a programmable gain with measurement ranges as small as 0,.256v. The constants for selecting these ranges are defined in the .h file. https://github.com/adafruit/Adafruit_ADS1X15