Language usage refers to the rules for making language, i.e. the structures used. It can be compared to use, which considers the communicative meaning of language.
Example
The usage of the third conditional is: If + past perfect + would + present perfect
In the classroom
Using an inductive approach is an effective way to get learners to explore usage. They can for example identify the target language in a text and then look at its usage and use.
Further links:
https://www.teachingenglish.org.uk/article/planning-a-grammar-lesson
https://www.teachingenglish.org.uk/article/teaching-grammar-inductively-catherine-walter