13.20 NAT with Route Maps - Why "overload" is used?

Hi Experts,

Any idea why "overload" keyword is used under this config?
IMO, overload is not needed as the NAT is dynamic NAT one to one.

ip access-list standard SOURCE
 permit 150.1.2.2 0.0.0.0

route-map R2>S0/0 permit 10
 match ip address SOURCE
 match interface s0/0

route-map ANY>S0/0 deny 10
 match ip address SOURCE
 match interface s0/0
route-map ANY>S0/0 permit 100
 match interface s0/0

ip nat pool POOL1 155.1.23.200 155.1.23.200 prefix 24

ip nat inside source route-map R2>S0/0 pool POOL1 overload

PS: This config is exactly the same with SG's except the name of route-map and NAT pool have been changed.