NEC 690.31(B)(1) Identification:
"... The permanent marking means for nonsolidly
grounded positive conductors shall include imprinted plus signs (+) or the word POSITIVE or POS durably marked on insulation of a color other than green, white, or gray. The permanent marking means for nonsolidly
grounded negative conductors shall include imprinted negative signs (—) or the word NEGATIVE or NEG durably marked on insulation of a color other than green, white, gray, or red. Only
solidly grounded PV system dc circuit conductors shall be marked in accordance with
200.6."
The sol-ark like most modern systems, the PV circuits are nonsolidly grounded.