I understand that you can create a separate yang file (Something like a textual Convention to store syntax values for MIBS)and import it into another yang file to make the data more organised and structured, but I can't seem to understand what the include statement does differently?
Does it "import" the entire file into the file that's including it - and if so would this be read before the file including it...?
Please help :)
YANG relies heavily on a concept known as "namespaces", which stems from XML naming conventions. Each namespace has a unique resource identifier and allows definitions (in different namespaces) to have same names at same definition levels while avoiding name clashes. When you define a YANG module, you are actually defining a namespace.
An import statement is used to access definitions from a foreign namespace (another module), while an include statement introduces a mechanism that allows a single namespace (single module) to be logically split into several files, conveniently named module and submodules. For includes, there is always exactly one module file, which includes all submodule files that belong to it. A submodule may only belong to a single module and may not be imported (directly). To an importing module, a module that includes submodules, looks like a single entity. Submodules may include eachother, but with YANG version 1.1, this has become unnecessary, since a submodule immediately gains access to all definitions in all submodules and the module they all belong to. In YANG version 1 you had to explicitly include a submodule to use definitions from it in another submodule, while never being able to access definitions in the module to which they belonged.
An import does not "inline" definitions into the importing module, while an include does exactly that. Importing a module gives you access to its top-level definitions (typedefs, groupings, idenitites, features and extensions) and allows you to use schema node identifiers that identify nodes in the imported module (for the purpose of augmentation and deviation, for example).
Definitions from a foreign namespace are always accessed via a prefix, which are part of an import statement's definition. Definitions that come from includes do not need to be prefixed when used, and if they are, are prefixed with the including module's (or submodule's) prefix.
YANG "compilers" usually process these files when they hit either an import or an include statement. They need to process them in order to be able to resolve definitions in body statements of the defining module. That is why these statements are required to appear in a module's header section.
There is an entire section in YANG specification dedicated to modules and submodules, where you can read more on the topic.