For the first slot we have 6 possibilities.

For the second slot we have 5 possibilities (we can't put a tab with the same number as in the first position).

For the third slot we have 5 possibilities (we can\t put a tab with the same number as in the second slot)

..........................................

For the sixth slot we have 5 possibilities.

So, using the product rule of combinatorics, we have possibilities.