Dear Dezheng.Zhu,
As mentioned in the MLife User’s Guide, TypeLMF can be set to explicit fixed mean value or the keywords “AM” or “WM” can be used to specify the aggregate mean (AM) across input files or weighted mean (WM) based on the specified Weibull distribution.
The old FAQ was lost with the loss of the old NWTC Information Portal, but the specific FAQ regarding TypeLMF and LUlt was posted on our forum–see my post dated Jun 14, 2020 in the following forum topic: MLife Fatigue DEL Calculation - #29 by Jason.Jonkman.
Best regards,