SAMPLE_ROOT macro appearing, I don't know why?

[ambalboa]

I have a 'Set Macro' step which would get the version number of a .exe file, updating the VER_NUM global macro. On one project file, I can run the step without it prompting me to define the SAMPLE_ROOT temporary macro; on the other, it prompts me for a value for that macro. Why and where did this macro come from? It's not the script, because I click on cancel multiple times to get rid of the SAMPLE_ROOT macro window, it builds successfully, and I check the version value in VER_NUM and it's the one I wanted. Can someone explain what is happening? Thank you.