The Layout UVs action automatically moves UVs so they don’t overlap in texture space.
It is not absolutely necessary to keep UV pieces separate. For example, you may want to overlap UVs so different faces use the same region of a texture. In general however you should keep pieces separate for convenience and clarity.
Subdiv Surfaces > Texture > Layout UVs >
The default settings give the best results in most situations.
Maya puts a bounding box around each piece and lays out the pieces so that the bounding boxes are very close together. If the pieces end up positioned exactly next to each other, two UVs on different pieces can share the same pixel and when texture painting, overscanning can also cause the paint to spill onto the adjacent piece.
To avoid this situation, ensure that there is at least a pixel between the bounding boxes by selecting a spacing preset from this menu. Select a preset that corresponds to your texture map size. If you don’t know the size, select a smaller map, which will result in a larger spacing between adjacent pieces in UV space. (The smaller your map in pixels, the bigger the UV spacing must be between bounding boxes.)
Select Custom to set the size of the space as a percentage of the map size (in the Percentage Space box).