Explanation of "Do not pass multiple-fixed effects with intersecting groups"

[DeFilippis]

Hey team,

I notice in the documentation for the fixed_effects flag for lm_robust it mentions the following: "Do not pass multiple-fixed effects with intersecting groups." I can't find any documentation on what is meant by an intersecting group or why multiple fixed effects cannot be added.

For example, if I want to add a fixed effect for country and a fixed effect for company, would I not be able to do so if the same company exists more than once in the same country? Was just hoping for clarification on this point.