Constant MAX_GRINDING_BITS 

pub const MAX_GRINDING_BITS: usize = 32;

Largest proof-of-work difficulty the transcript accepts, in bits.

The sampler masks what it returns to 32 bits. A larger difficulty could not be expressed, and would silently weaken to this one. A 32-bit grind already costs about four billion hashes, so the cap never binds in practice.