If I’m running a LF study (motor impact starting) and my sources are all generators (defined as SB), what source impedance is the LF study options dialog referring too?

Answer: Subtransient impedance defined on the ANSI contribution subview.

Notes: Question came about because the Brown book recommends using the transient impedance for this calculation when running impact motor starting studies.
See page 245, section 9.6.2 of IEEE std. 399-1997 (Brown Book)