Yes, VSUBS is the name that magic gives to the substrate node if it is not labeled something else. In your typical nMOS layout, the bulk of the transistor is connected to the substrate, and this should be tied to ground and is probably labeled with something that overrides the default name. In your typical pMOS layout, the transistor bulk connection is the nwell, but if there is not a larger circuit around the device, then probably your substrate is not connected to anything, so it gets the default name. This behavior will go away if, for example, you add a substrate contact below your pMOS device and label it, say, VSS.