It is possible to suggest, in score or tablatures, which fingers should be used for picking or strumming the strings. These suggestions can be freely adjusted to the physical properties and individual experience of the player.

The best fingering is the one that causes the least movement of the hands and is therefore most comfortable. Any rule that does not have convenience as its purpose is bad. … The player has to look very closely at the following notes and then to choose the fingers.1)

Fingering numerals

frethand

For the fret hand, Arabic or Roman numbers are used:

Fingering Letters

Strumming hand

For the strumming hand, Spanish or German letters are used:

SpanishFingerGerman
PthumbD
Iindex fingerZ
Mmiddle fingerM
Aring fingerR
Q or Clittle fingerK

1) Johann Ernst Häuser: Guitarre-Schule. Basse 1835, p. 6
2) or T, if 0 is used for the open string (which is the rule with the ukulele).