Example:
1. YZF-YYC/YVR/YEA-UA/LH/AC-SFO/SEA/PDX/LAX/HOU/DFW/DEN/CHI-
UA/LH/AC-WAS/PHL/NYC/EWR/DTT/CLE/CHI/BOS-UA/AC-ATH-AC/A3-
MIL
Means that you can fly from YZF to YYC or YVR or YEA and then on UA LH or AC to SFO or SEA or PDX or LAX or HOU or DFW or DFW or DEN or CHI on UA LH or AC etc.
You can also skip any of the cities above. So you could fly YZF-YVR-EWR-MIL using the above routing options.
The above routing does not mean that there are flights that will cover all these routings - BUT if there are flights between any of the cities, then providing there is appropriate space you should be able to book it.