What is the "Detailed format classification" of the ISO 6983 (G-Code spec)
This is simply the formalised definition of the syntax, so that a parser can be written to interpret any legal G-code. Without this, there is ambiguity - not in the general operation, but in the bounds of what is 'legal' and what should be rejected.

Taking the example X+053, this is not a position of 53 units, it is:


Address X
Sign is required (in this implementation)
Leading zeros can be ommitted
Up to 5 orders of magnitude before the decimal
Up to 3 digits of precision after the decimal


This means that X can range from +99999.999 to -99999.999 - an interpretation which matches the verbiage at the bottom of the page.

As hinted at in the text, it is possible to use various fixed precision number systems (think using integer milimetres rather than decimal metres), and also to anchor at the most significant bit, so 001 could represent 00100.000. It may help to remember that these specifications were designed in the days of dedicated hardware rather than general purpose computers, when the standard portable storage medium held ~600 kB, and relate to actual machines that had been developed long before that.