Reference fields are similar to many-to-one fields, except that they allow the user to select the model to link to.

The target model is selectable from a list provided by the selection attribute. The selection attribute must be a list of two element tuples, where the first is the model internal identifier, and the second is a text description for it.

Here's an example:

[('res.users', 'User'), ('res.partner', 'Partner')] 

However, rather than providing a fixed list, we can use a model list configurable by end users. That is the purpose of the built-in Referenceable Models available in the Settings | Technical | Database Structure menu option. This model's internal identifier is res.request.link.

Our recipe started with providing a function to browse all the Model records that can be referenced to dynamically build a list to be provided to the selection attribute. Although both forms are allowed, we declared the function name inside quotes, instead of a direct reference to the function without quotes. This is more flexible, and for example, allows for the referenced function to be defined only later in the code, which is something that is not possible when using a direct reference.

The function needs the @api.model decorator because it operates on the model level, not on the recordset level.