explain the project-join normal forma relation r

Question

Explain the Project-Join normal form

A relation R is in PJNF along with respect to a set of functional dependencies D if for all join dependencies of the outline *(R₁, R₂, ...,R_n) and R =  R₁ U R₂ U ..., UR_n at least one of the subsequent holds:

 *(R₁, R₂, ...,R_n) is trivial

 Each R_i is a superkey of R