Section 2(1)(d) in The Central Electricity Regulatory Commission (Indian Electricity Grid Code) Regulations, 2010
(d)"Available Transfer Capability (ATC)" means the transfer capability of the inter-control area transmission system available for scheduling commercial transactions (through long term access, medium term open access and short term open access) in a specific direction, taking into account the network security. Mathematically ATC is the Total Transfer Capability less Transmission Reliability Margin;