Meter Based PMs should have a tolerance in meter units which will cause the work order to be generated early

Time based PMs have a Lead Time that allows work orders to be generated early to provide time to assemble materials, external services or tools necessary to perform the work. There is a field on the Preventive Maintenance Meter tab called Generate WO Ahead By (PMMETER.TOLERANCE) which according to the original design was for information purposes. This field should be used to generate a work order earlier than the due date. The number of meter units would be multiplied by the average units/day to determine a Lead Time (Days) on each meter. This would be a non-persistent (and read-only) field to help understand which meter, or if mixed meter/time based was the driver for the generation of the work order. 

If no time based frequency then the meter with the greatest Lead Time would be added to the Slack Time to determine whether to generate the work order early.
If mixed time and meter based frequencies then the greater of any Meter Lead Time or if Lead Time Active then the Lead Time (Days) (PM.LEADTIME) would be added to the Slack Time to determine whether to generate the work order early.

The use of the Generate WO Ahead By should not reduce the Units To Go or affect the Estimated Next Due Date because this pulls forward the Target Start date on the next generated work order. This is how it is working today (MAS 9.0, 9.1), but this is having the same effect as reducing the frequency by the same value. For example, if average units/day is 10 and meter frequency is 100 and Generate WO Ahead By is 10 then a work order is generated every 9 days instead of 10 days. It would be the same as if you set the frequency to 90 instead of 100 and left Generate WO Ahead By as 0. The result is the current use of Generate WO Ahead By is causing more work orders to be generated each year than the frequency would suggest. 

There is a case TS013533562 and APAR DT248195 which references this issue. The APAR should now be changed to remove the code that uses the Generate WO Ahead By to reduce the Units To Go.

The Generate WO Ahead By field should have no impact on the Estimated Next Due Date or the Earliest Next Due Date. 

The Generate WO Ahead By field should have no impact on PM forecast dates.

The Generate WO Ahead By on a Meter based PM in a PM hierarchy would be triggered by the meter in one of the PMs in the hierarchy which had the greatest value in the new non-persistent field Lead Time (Days).