What symbol must replace the question mark in order to get a 7-letter word that describes this puzzle?
Note: Ignore the different size/thickness of the symbols (using phone)
Hint 1
It's based on something very simple.
I believe the answer is
SQUARES and the symbol that goes on the top is 00.
Reason
These are simply reflections of square numbers 1,4,9 along the y axis so the beginning of the sequence has the be the perfect square before 1 which is 0 and reflecting 0 we get 00.
As for the title
What is a box but a square.