# In linear optimization, what does “AP” stand for?

I am learning algorithms, and there is a chapter which uses linear optimization methods to solve a matching problem. This is the problem definition:

I find the abbreviations AP for the constraints of the primal problem and APD for the constraints of the dual problem confusing. Could somebody please explain what they stand for in this context?

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$\text{AP}$ stands for assignment problem or allocation problem. So $\text{AP}_{D}$ is the dual problem.