RMS

The column titled #RMS in the table associated with the key _Sparse SABR Paramsis mandatory and contains elements of type Number.
Each element represents the root-mean-square error achieved during the optimization routine run over the available swaptions with the respective (tenor, expiry) pair.
The error is in the same units as the market volatility being used.
So if Black vols are used, a maximum approximation error of 0.03 means that the calculated SABR parameters α, β, ν, ρ are such that the SABR-implied swaption Black vols differ from the supplied market Black vols by amounts that do not exceed 3% (absolute difference).
Obviously the smaller the error, the better the market fit.
Note a very high maximum approximation error may coexist with a low root-mean-square error, which would indicate that most strikes have been well matched, but at least one strike had a bad fit.