Difference between iso_level3_shift and latch

What is the difference between ISO_Level3_Shift and ISO_Level3_Latch, for example, in the level 3 configuration file I found (in archlinux and many other distributions) in / usr / share / X 11 / xkb / symbols.

For example, I added this entry to the user keyboard I'm working on (for logicians, don't worry about it, ⊥ → awesome).

// make right control do level3 shift stuff
default partial modifier_keys
 xkb_symbols "rctrl_switch" {
  key <RCTL> {
    type[Group1]="ONE_LEVEL",
    symbols[Group1] = [ ISO_Level3_Shift ]
    };
  include "level3(modifier_mapping)"
};

It works as expected (holding the right control, I get access to level 3 characters, for example, mainly the unicode code numbers that I have for the us-intl layout option), but I wonder why some of the entries use ISO_Level3_Latch , and what does it mean.