mark the spot

This passage is the continuation of that of the question

Could you please let me know what does 'mark the spot' mean?

There were two in there. He had forgotten, lost track; he could
not now remember whose wands these were; he seemed to remember
wrenching them out of someone’s hand. He selected the shorter
of the two, which felt friendlier in his hand, and pointed it at the
rock.
Slowly, under his murmured instruction, deep cuts appeared
upon the rock’s surface. He knew that Hermione could have done
it more neatly, and probably more quickly, but he wanted to mark
the spot as he had wanted to dig the grave. When Harry stood up
again, the stone read:
HERE LIES DOBBY, A FREE ELF.