A natural branch of international maritime flag language was flags being used for naval purposes.
“Use of flags outside of military or naval context begins only with the rise of nationalist sentiment by the end of the 18th century; the earliest national flags date to that period, and during the 19th century it became common for every sovereign state to introduce a national flag.”
This is noteworthy, as our understanding of flags is almost exclusively in nationalistic terms. The first flags we think of are of countries, and that doesn’t need to be backed up with a library of research. National flags are synonymous with even the word “flag.”
These flags borrow and take inspiration from each other. Over time, we’ve become a world of thousands of unique national and state flags.
A small subset of national flags are civil flags, which are different versions of national flags that are flown by civilians on non-governmental buildings or crafts.
Another, much, much larger subset of national flags are war flags. For most countries, every branch of their military has a specific flag, and within the military, different units of the military have their own specific flags.
And even combining nations, there are international flags, flags that serve as means of identification for several nations. These would include flags for the United Nations and bodies such as the Olympics or the Paralympics.
There are also religious flags. These flags, as the name clearly states, are flags that are used to identify either the governing body of a religious body or the religion itself.