1. The Creator is used to trigger the workspace when there is no other input data available. For example to create a workspace that triggers a web service, you could use a Creator + HttpCaller.
2. The GeometryCoercer is used to force the geometry interpretation (when possible), for example to force FME to interpret polygons as closed line strings (perimeter).
3. The Aggregator/Deaggregator are for composing/decomposing multi-part geometries.

There's also a lot of learning material available for free here:

• https://www.safe.com/training/recorded/
• https://community.safe.com/s/academy

Thank you David! thanks for your help.

I still kind of confused on Geometry Coercer. What do you mean by "geometry interpretation"? Does it mean to read the geometry and "extract" different geometry types, such as closed line strings?

Thank you.

It's a rather esoteric transformer, so unless you've stumbled upon a case where it's really needed, you probably won't The documentation describes it rather well:

This transformer is sometimes used to have area features treated as though they were linear features, either because some later processing requires lines, or the destination format represents lines different than polygons and the linear representation is desired.

Source: https://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_Transformers/Transformers/geometrycoercer.htm

I see... thank you for your help!