Stationary Shield is avalible to defending Tech classes in "weapon" Siege maps.
This stationary energy field can be dropped at any location, at which time it deploys a large flat shield that can block enemy fire. Allies can pass through the shield, but enemies cannot.
The deflector shield will disappear after expending its power supply or after enduring a certain amount of damage.
The Shield is extremally usefull tool. Some Technians in Siege may sometimes perform so called "Shield spam". It consists of spawning- putting your shield in the main door and than suiciding, respawning and putting the shield again. This is very annoying technique, which mainly results in permanent blocking of the attacking team.
Because of this the Shield spam is usually forbidden and roughly punished on most servers.