Rule 5.09 (a )(1) Comment (Rule 6.05(a ) Comment )
: A fielder
may reach into, but not step into, a dugout to make a catch, and
if he holds the ball, the catch shall be allowed. A fielder, in
order to make a catch on a foul ball nearing a dugout or other
out-of-play area (such as the stands), must have one or both
feet on or over the playing surface (including the lip of the
dugout) and neither foot on the ground inside the dugout or in
any other out-of-play area. Ball is in play, unless the fielder,
after making a legal catch, steps or falls into a dugout or other
out-of-play area, in which case the ball is dead. Status of run-
ners shall be as described in Rule 5.06 ((3)© Comment
(Rule 7.04 © Comment).
Catch Comment
: A catch is legal if the ball is finally held by
any fielder, even though juggled, or held by another fielder
before it touches the ground. Runners may leave their bases the
instant the first fielder touches the ball. A fielder may reach
over a fence, railing, rope or other line of demarcation to make
a catch. He may jump on top of a railing, or canvas that may be
in foul ground. No interference should be allowed when a
fielder reaches over a fence, railing, rope or into a stand to
catch a ball. He does so at his own risk.
If a fielder, attempting a catch at the edge of the dugout, is
“held up” and kept from an apparent fall by a player or players
of either team and the catch is made, it shall be allowed.
2017 Official Baseball Rules_2017 Official Baseball Rules
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The referenced part about the runner(s) basically says if the fielder makes a legal catch and carries the ball out of play (i.e., he is completely in the stands/bullpen/dugout), each runner advances one base. The second part sounds like the fielder would be allowed to to use the top of the wall to propel himself up to make a catch (if he could figure out a way to keep a foot still in play, not sure the laws of physics would allow that).