What is the meaning of “`” in the picture?
In the C3 type DSL, a backtick is used as an escape character when you need to use a DSL keyword as an identifier (e.g. a field name or a parameter name). In the example you provided,
set is being used as the foreign key field name, but since
set is also a collection type, a backtick is included to tell the DSL parser to treat it as a foreign key field name. I believe the backtick is mostly a legacy feature since the DSL parser has been improved over the years to intelligently distinguish identifiers from other AST nodes.