what is ERC

Electrical rules checked in this step are usually limited to errors in connectivity
or device connection. Examples include the following:
• Unconnected, partly connected, or extra devices
• Disabled transistors
• Floating nodes
• Short circuits
• Special checks not checked elsewhere (i.e., antenna rules)
As a subset of the LVS check, an ERC generally executes more quickly
and therefore is useful to accelerate debugging problems such as a VDD-to-VSS
short circuit.
Re: what is ERC

one more point is added:
The # of contacts for a particular width device since it has to drive the current from drain or source to target.