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 or equal to operator.

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

say (1,2,3) (<=) (2,3,1); # OUTPUT: «True␤» 
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 subset or is equal to $b, else False. More information, Wikipedia definition.