You can tell which way up the crates have to be because they all say 'TOP'.
It should read, "You can tell which way the crates have to be because they all say 'TOP'."
It means that you can tell which side of the crate is the top side because it has the word "top" on it. You can tell how to stack the crates (which way the crates have to be) based off of where the word "top" appears.
Can you tell me what "based off" means in this sentence?
Could you give me some examples of the phrase "based off" with the similar meaning?
There isn't one. It is an incorrect phrase, apart from something like : The warship is based off the coast = the warship is stationed somewhere in the sea near the shore.