"top of the stack" means the upper end of the memory region that you have reserved for the stack. "Memory border" means the same. For example, if the SRAM ends at 0x23FF, and you initialize the stack pointer to 0x2400, and push some values, and later use POPM so the the stack would be empty afterwards, then the CPU tries to read not only the words at …, 0x23FC, 0x23FE, but also the word at 0x2400, which is not a valid address.
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