Subsidiary COA:
Company-Account-Department

Parent COA:
Company-Account

In Segment Rules mapping you will simply ignore Department in your mapping. Segment Rule mapping is based on Parent segments. All parent segments must have a mapping.

In Account Rules mapping you will map subsidiary combinations with distinct department values to a single parent account:

01-1000-10 -> 20-1000-00
01-1000-20 -> 20-1000-00
01-1000-00 -> 20-1000-00

Lets wait for next few videos till you see the demos on creating mappings. Then see if your question still remains outstanding.