I assume you mean Assumed Temperature?
For airplanes without the aspirated TAT option, OAT is entered on the N1 LIMIT page of the FMC CDU. For airplanes with the aspirated TAT option, OAT entry is not required, but if OAT is entered, the entered value is used for takeoff settings instead of the sensed value.
Then there is the Assumed Temperature (Flex equivalent) - Regulations permit the use of up to 25% takeoff thrust reduction for operation with assumed temperature reduced thrust.
Maximum assumed temperature can be found in FCOM, it is a function of OAT and pressure altitude, 73C at -1000' with OAT 55C, in the ISA range from 69 to 51 is allowed...
To find the maximum allowable assumed temperature enter the Maximum Assumed Temperature table with airport pressure altitude and OAT. Compare this temperature to that at which the airplane is performance limited as determined from available takeoff performance data. Next, enter the Maximum Takeoff %N1 table with airport pressure altitude and the lower of the two temperatures previously determined, to obtain a maximum takeoff %N1. Do not use an assumed temperature less than the minimum assumed temperature shown. Enter the %N1 Adjustment table with OAT and the difference between the assumed and actual OAT to obtain a %N1 adjustment. Subtract the %N1 adjustment from the maximum takeoff %N1 found previously to determine the assumed temperature reduced thrust %N1.
Apply %N1 adjustments as provided when applicable.