Piece rate is a multiplier working similarly as SMV, but while the SMV works by giving incentives on the speed of the operations the piece rate works by giving incentives on how many pieces/products are produced.
For example, on very short operations that are done in quick succession the SMV might be so low it looses relevance. In that case the piece rate can be used instead to count number of produced pieces.
Piece rate is also affected by any workmode factor that applies to an operator.
Please also see Incentive Mode.