How is the PK parameter AUC_inf_tD1 calculated?

[teutonicod]

Dear all,

I have a PK-Sim model with repeated doses, and in the Pharmacokinetics Analysis there is the first AUC calculation, AUC_inf_tD1. In the manual it is mentioned that is the extrapolation of the AUC " from the first data point to infinity", but this is not very clear. Could you let me know how this calculation is performed? Below a screenshot from the manual.

Thanks,

Donato