In the Data Specifications page, it says that some quantities in the stellar_circs supplementary data file are calculated by using only particles within 10R_E, while others are calculated using all particles assigned to the subhalo. But I can't find a definition of R_E anywhere. What is it?
My guess would be the effective radius (half light/mass radius)
Yes R_E would be an abbreviation for effective radius, typically referring to "half-light", particularly when used observationally.
In this case, it refers to the stellar half-mass radius (i.e. SubhaloHalfmassRadType).