Sequential vs. Functional

Keras Models: Sequential vs. Functional

There are two ways to build Keras models: sequential and functional.

The sequential API allows you to create models layer-by-layer for most problems. It is limited in that it does not allow you to create models that share layers or have multiple inputs or outputs.

Alternatively, the functional API allows you to create models that have a lot more flexibility as you can easily define models where layers connect to more than just the previous and next layers. 

Reference:
https://jovianlin.io/keras-models-sequential-vs-functional/