Purpose : Performance Efficiency Rate Key
Performance efficiency rate keys are assigned in the Basic data screen of the work center to thestandard values. The performance efficiency rate key is the ratio between an individual‘s actual output and the planned average output
Internally areference indicator is set for the performance efficiency rate key. This means that the performance efficiency from the work center is transferred to the operation and cannot be changed there. If you do not enter a performance efficiency rate key, the system assumes 100% efficiency.
The standard values in routings are always based on a performance efficiency of 100%.
You define performance efficiency rate keys in Customizing for Work centers by choosing Define key for performance efficiency rate.
The term efficiency must be quite clear to you, so how it applies here is
SAP uses it w.r.t to scheduling & costing.
Eg: Std value key - SAP1 has Setup, Machine & Labor. So it is a given that man or machine cannot work at 100% efficiency, so you define a realistic value & assign the efficieny key here.
What this does is, when the scheduling is done, it takes this efficiency key into calculation, so if you have maintained 75% efficiency for setup & the time req is 1 hour, then it schedules the time required as 1 hr 15 mins & the same goes for costing as well.