# can virtual address map any on virtual memory on disk?

I was reading a textbook which says that:

If the valid bit is set, the address field indicates the start of the corresponding physical page in DRAM where the virtual page is cached. If the valid bit is not set, then a null address indicates that the virtual page has not yet been allocated. Otherwise, the address points to the start of the virtual page on disk.

My question is, does each page tabe entry(PTE) need to be match the same offset one in disk? for example, PTE 1 has VP 1 in physical memory, but can PTE 1 originally mapped to VP 3? or PTE 1 has to map VP 1, PTE 2 has to map VP 2, PTE 3 has to map VP 3 and so on?

if PTE x has to map VP x, then why when valid bit is not set then the address points to the start of the virtual page on disk? can't we use the offset x in PTE x to infer that it maps to VP x?