Overall Project Conventions
Often there are conventions that are part of the overall project. These may be simple rules about who or how the application is allowed to access the database. Also simple things like subdirectory structure, file naming conventions, and all items that ensure consistency between multiple developers.
Database Conventions
The database has many conventions, such as the use of id as the first field as the primary key, table naming conventions, field naming conventions, use of views, use of stored procedures. If you create a table, you do it by rules – these rules go here.
Coding Conventions
Code conventions can follow documented recommendations for variable, method, function, and file naming. Also where code is organized, use of folders in the code organization. If you code it and it follows any rules for consistency – those rules must be here.
UI Conventions
You don’t want every page and input screen looking different. You may establish a single CSS and enforce rules to use this for all screens. Any way you do it, the interface must be consistent and your documentation of this must be here.
Documentation Conventions
Even how code is documented must be a documented convention. Examples I use is all code is always in a specific font in the documentation and enclosed in a table.