The implementor's model describes the system internals used to implement the designer's model. The implementor's model includes details relevant only to programmers and others who develop the product.
For example, the designer's model might include a directory object that consists of people's names, addresses, office numbers, and so forth. The implementor's model of the directory object might consist of records in a file, with one record for each directory entry, or it could be a complex organization of multiple records from multiple files.
Users should be shielded from the complexity of the implementer's model; the designer's model should not convey the technical implementation details.