If not nil then the optimization method uses it for logging each optimization step. By default the Verbose property is nil meaning no logging is done. 

If assigned, stores Fun, evaluated at each iteration step. Optionally, you can also assign TOptControl object to the Verbose property. This allows the optimization procedure to be interrupted from another thread and optionally also allows logging and iteration count monitoring.

Log to Memo.Lines:

TMtxNonLinReg Properties