Documentation for infix (<), infix ⊂ assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix (<), infix ⊂ §

multi sub infix:«(<)»($a,$b --> Bool:D)

Subset of operator.

Returns True if $a is a strict subset of $b, i.e., that all the elements of $a are elements of $b but $a is a smaller set than $b.

say (1,2,3) (<) (2,3,1); # OUTPUT: «False␤» 
say (2,3) (<) (2,3,1);   # OUTPUT: «True␤» 
say 4 ⊂ (1,2,3);         # OUTPUT: «False␤» 

Language documentation: Sets, bags, and mixes §

From Sets, bags, and mixes

(Sets, bags, and mixes) infix (<), infix ⊂ §

Returns True if $a is a strict subset of $b, else False. More information, Wikipedia definition.