## refers to a uint type. If I write ## 16, that refers to a uint of 16 bits, so up to 2^16 - 1. There is also a type #, which is a shorthand for uint32. Let's display a few examples:

• ## 8 = uint8
• ## 64 = uint64
• ## 32 = uint32
• # = uint32

Now let's look at your case. ## 1 = uint1. As in one bit. In other words, ## 1 is a boolean.