Minecraft wiki entry for Universally_unique_identifier, associated text: Hyphenated hexadecimal: A hexadecimal representation of the UUID, with hyphens separating the different sections into individual numbers.
The hyphen are set to split the UUID into numbers of the format 8-4-4-4-12 with each number marking the number of hexadecimal digits fitting into the corresponding section. Further detail follows in the Technical aspects section.
An example of this representation would be f81d4fae-7dec-11d0-a765-00a0c91e6bf6.
Because every part is evaluated as an individual number, empty digits at the start of a section can be ignored.
For example, 00000001-0002-0003-0004-000000000005 can be evaluated as the same as 1-2-3-4-5.
Hexadecimal: The same as the hyphenated hexadecimal representation, but without the separation of the different sections.
An example for this representation would be 00000001000200030004000000000005, where it is impossible for the majority of empty digits to be removed as opposed to the hyphenated hexadecimal representation.
Most/Least: A separation of the 64 most significant bits from the 64 least significant bits. Each of the two numbers is stored separately and uses the Long data type in the game.
This format is deprecated as it was used before 1.16 and was entirely replaced by the Int-array format.
An example of this representation would be UUIDMost:-568210367123287600 paired with UUIDLeast:-6384696206158828554.
Int-array: A separation into four 32 bit numbers. Each part is stored in an integer array ordered from most significant to least significant.
An example of this representation would be [I;-132296786,2112623056,-1486552928,-920753162]