arcpy fieldmappings start/end text position(s) for the field map?

[TylerT]

Greetings,

I am studying up on field mappings and have a question. 

What are the "start/end text position(s) for the field map"?

Thank you,

Tyler

[DanPatterson]

the discussion and code example here...

Mapping input fields to output fields—ArcGIS Pro | Documentation

provides a use-case