In solidity, there are 2 ways to represent uint values.

• Decimal, 123
• Hexadecimal, 0x006B

And the number FIELD_PRIME = 0x800000000000011000000000000000000000000000000000000000000000001 is a valid uint256 value in hexadecimal format. Its not an address as @Satoshi mentioned. Its a positive integer value, which is what uint256 requires.

Thx