Constant DEFAULT_RATE_REFINEMENT_MAX_ROWS 

pub const DEFAULT_RATE_REFINEMENT_MAX_ROWS: usize = 1000;

The default bound on the rows the rate refinement may add (a safety bound against a rate that fails the test in many cells at once; not a tuned value): 1000 rows, so cells of 18 meV on average across the 18 eV of RATE_WINDOW_EV, far below the 0.1 eV step of the rate gate. A step in the rate splits one cell per level and is stopped earlier, when its cell is exhausted to rounding (module docs, “Rate refinement”).