Mesh volumes are specialised volume types which use a polygon mesh to define a volume. They are the only volume type in Particle Flocker which does not use a VolumeLocatorNode to perform rendering or editing operations.
Mesh volumes are defined entirely by a polygon mesh and can be any shape you want them to be, however the system does make an assumption that the mesh will be a completely closed volume shape. If you use a mesh which doesn’t form a complete volume such as a flat plane for example, you are likely to experience strange simulation results.
It should be noted that mesh volumes are much more computationally expensive to use than standard primitive volumes and so will have a detrimental effect on simulation performance in Maya. It is recommended that you keep polygon counts as low as possible to keep simulation performance higher.