Documentation for method capture assembled from the following pages:

Class: Parameter §

From Parameter

(Parameter) method capture §

Defined as:

method capture(Parameter:D: --> Bool:D)

Returns True for parameters that capture the rest of the argument list into a single Capture object.

sub how_many_extra_positionals($!, |capture) { capture.elems.say }
how_many_extra_positionals(0, 1, 2, 3);                        # OUTPUT: «3» 
say &how_many_extra_positionals.signature.params[1].capture;   # OUTPUT: «True␤»

Like raw parameters, Capture parameters do not force any context on the values bound to them, which is why their sigils are only used in declarations.