Basically, Available Transfer Capability (ATC) is a measure of the transfer capability remaining on a transmission grid above already committed uses. Hence ATC distribution among suitable candidate buses of a given network can be used to locate a new generation facility. This ATC distributions alone might not be adequate, hence it can be further verify with margins such as locational marginal pricing (LMP) which considers the cost of generation from a particular location.

Transmission grid performance can be measured using ATC values relative to power losses incurred as a result of a given transfer direction. Hence, Available Transmission Transfer Efficiency, a novel termed used for technical transmission grid performance indicator unlike the financial key performance indicators (KPI).  

Article International Journal of Engineering Sciences A Comparative ...

Article Available Transfer Capability (ATC) as Index for Transmissio...