Exposure groups and combining data

How is the exposure group ID used when combining data sets? Do I need to make sure to add an offset if I know I will be combining?

For example, If I run Global CTF Refinement on two datasets from the same microscope but with different beam conditions, and the same file conventions and exposure group IDs (e.g. 0 - 9), will there be 9 groups or 18?

As of v2.15, an offset must always be added if datasets will be combined. Exposure IDs are global.