PPFD: It is a value that explains how many photons in specific range（ it’s from 400- 700 nm for plant growth ) of the visible light spectrum (photosynthetic active radiation or PAR ) that fall on a square meter of target area per second. The unit is µmol/m²s. Intensity mainly affects the overall yield of your crop. In general, the minimum PPFD values needs for plant growing is 53µmol/m²s.
YPFD: Because PPFD is just calculated from a known spectrum but not related to plant light response. To check how efficient the light for plants, have to weight the value by required PAR action spectrum to get Yield Photon Flux Density (YPFD). The Value will change when the reference PAR action Spectrum changes.