The DS defines a Data Structure.
The S defines a single field.
A Data Structure is a group of fields.
Here is a sample:
d inRecordDS ds
d inData 94a
d inkey 39a
d recType 1a overlay(inData:1)
In my code, I can reference inRecordDS which will be 133 characters long
Or I can reference InData which will be just the first 94 characters
Or I can reference recType which will be just the 1st position of inData