Why Does Iden3 Use 253-bit Slots for Claims

Question

I'm exploring the Iden3 protocol, and I noticed that Iden3 designs its claim slots to be 253 bits in size, rather than the more common 256-bit size. I'm curious about the specific reasoning behind this choice. I observed that in the AuthBJJCredential type of claim, the BJJ public key is placed in the first two slots of the claim's value. Does this imply that the uint8 value at the slot index 31 cannot exceed 32? I'm trying to understand the implications of this design choice on the data structure and the constraints it imposes on the cryptographic elements of the system