How to understand the comments about fat_arch_64 in 'mach-o/fat.h'

Question

Recently, I'm learning some materials related to Mach-O and noticed the comment

When a slice is greater than 4mb or an offset to a slice is greater than 4mb then the 64-bit fat file format is used.

in 'mach-o/fat.h' about struct fat_acrh_64

I know we should use the value of fat_header.magic to decide use fat_arch or fat_arch_64 to parse the data in a fat file.

But I am confused about the comment, why the limit condition is 4 Mb and what's the slice/offset in the comment means?

Here is the screenshot for header file:

Answer 1

The comment is wrong and you can find counterexamples in the wild.
This is from an M1 Mac:

% otool -f /usr/sbin/php-fpm
Fat headers
fat_magic 0xcafebabe
nfat_arch 2
architecture 0
    cputype 16777223
    cpusubtype 3
    capabilities 0x0
    offset 16384
    size 14061120
    align 2^14 (16384)
architecture 1
    cputype 16777228
    cpusubtype 2
    capabilities 0x80
    offset 14090240
    size 14110192
    align 2^15 (32768)

As you can see, both size and offset are in excess of 14MB.

A reasonable guess would be that they meant 4GB, which is the limit of what a uint32 can represent.