maximum
The most of the realized range that fitting will leave beyond each end, as a fraction of that range.
Without a cap the fraction is 1/(n-1) for n alternatives, which is half the range at each end when there are three of them. Fitting would then treat three alternatives as spanning a third of a domain built almost entirely out of room left over, and every value would sit within [0.25, 0.75] however good or bad the alternative. The default cap of a quarter keeps the widest case to [0.167, 0.833] and binds only below five alternatives; at five it is exactly the uncapped fraction, and above five it does nothing at all.
Raising it above one half disables the cap, since one half is the largest fraction the uncapped estimate ever produces.