This mask can be used with a 3D File External referenced 3D file (.fbx and .obj) Read More with multiple objects in it, grouped in one mesh or with a Particles Simulate a particle system Read More 's group.
Their function is to apply a 3D Modifier Modify a 3D geometry Read More or a Field Defines a force-field in 3D space Read More only on a specific object or a particle Group. Groups of particles can be defined through Particle Rule Defines the behaviour of particles Read More .
On the example below, you see the white particles emitted in the Group [1]. These particles have an "Increase Group" action ("Set Group" action in menu) that changes their Group to [2].
The Gravity field has a Group Selector Mask set as Single Selection on Group [2], which means that the gravity field will only have an effect on the second Group.

3 mode of selection are available:
- Single selection: only the group set in the start parameter will be affected.
- Range Selection Block: all the group between Start and End will be fully affected.
- Range Selection Progressive: the lower group will be fully masked while the higher group will be fully affected, with a gradual progression in between.