Doors and shutters in Umbrella Corps can be held shut when a player is next to them.
This can be useful, but it's also a two-edged sword as there's a way around it.

There's small gap at the bottom of doors and shutters,
and this can be exploited with a well aimed gun shot or grenade lob.

Even completely closed areas have gaps or meshes on the ceiling or the floor that you can use to shoot through,
nowhere is completely safe so remember to learn the map layouts and make use of every nook and cranny!