wonder about redirect-gateway subnet/switch-mode

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quoted from http://www.tinc-vpn.org/examples/redirect-gateway/
"In fact, one does not have to set the VPN_GATEWAY variable at all. In
switch mode, the gateway routes are necessary"
i don't get it. it's is too short to understand. is VPN_GATEWAY set by
tinc in switch mode? and why is "In switch mode, the gateway routes
are necessary" mentioned here suddenly?

afaik, Subnet is announce by node itself to other nodes. that mean i
cannot set, for example. subnet=1.2.3.4/32 in hosts/X within nodeA,
while subnet=5.6.7.8/32 in hosts/X within nodeB
so, is it correct that i cannot have 0.0.0.0/0 for *two* gateway node
as in http://www.tinc-vpn.org/examples/redirect-gateway/ ?
mode=switch/hub instead of router have to be used in this case ?