[@Sep] The idea is you can choose from the list which weapons your Jaeger wants to take, instead of the standard twin arm-mounted [url=http://pacificrim.wikia.com/wiki/I-19_Plasmacaster]I-19 Plasmacaster[/url] and chest-mounted [url=http://pacificrim.wikia.com/wiki/WMB2x90_Anti-Kaiju_Missile_Launcher]AKM[/url] which would be standard on most of the Jaeger (the Shiva would just have four Plasmacasters to start with, as it can't fit the chest-mounted weapons due to its larger power core.)

The option to 'refit' your Jaeger between missions is also available, but only as long as there's time within the story for the work to be done. If you suddenly hear that there's a Kaiju heading for Tokyo then you'd have to leave right away, and wouldn't have a chance to change any of your weapons (although the weapons you already have fitted would be fully charged and reloaded with required ammunition.)