What is the isBis field for?

When I click on the 'Edit comic information' for an issue, in the 'General Info' section of the metadata I see a field labeled 'Bis' in between issue # and count. It's a boolean checkbox.

I'm curious, what is this field meant to track?

Recently started using the yacreaderlibrary software to manage my library of 40,000 cbz files, nice reader and a friendly community here.

It is used when two issues have the same issue number but one of them is supposed to be read after the other. This will be replaced with better issue numbers support, so you can have things like 10, 10.1, 10.2....