Chain table structure in pjlib

The linked list structure of pjlib differs from the common linked list structure, as shown in:

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Figure 1: General linked list Structure

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Figure 2: linked list structure in pjlib

We can see that the common two-way linked list node contains the data domain, while pjlib contains the data domain containing the linked list node. Generally, the linked list pointer field is the pointer of the linked list structure, while pjlib is the pointer of the data structure. I have not yet realized the advantages of this structure. I may have to understand it slowly, but my understanding of the chain table header is somewhat confusing. The linked list header is a separate list, And Prev and next point to the data structure containing the list. This structure is always strange to me.