From type/Parameter

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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.