Paolo, perhaps we are all agreeing over a language barrier?
I agree with Ed.
“Fingered legato” is a term invented by the Sound Library creators to describe how some of their samples transition from one note to another (see SPITFIRE Solo Strings Manual page 11). It has no connection to a particular notation. And in that world “Sul D” patches are just all sampled from an instrument’s D-string.
In the notation world, Sul G, G-Saite, Sulla corda Sol, Sul IV or even just IV___ is an instruction to play on a particular string. Sometimes this is for the ‘tone’. More often it is to ensure harmonics sound correctly, or is an editorial suggestion to help the player with fingering. To see examples of all these uses I suggest you consult any edition of the Paganini 24 Caprices, Op1 (download from ISMLP).
Beware, the IV format is not “transportable to any other instrument”. Specifying IV on a Violin is the equivalent of specifying III on a viola. However, Sul G is transportable! Similarly Sul C is OK on both Cello and Viola, but meaningless on a Violin.
In my 50+ years of playing I have never encountered the instruction “sulla corda” on its own, without a string being specified (but I accept, that might be because I have led a very sheltered life).