Russ Hinkley addressed this issue in the FT Chat, albeit somewhat indirectly.

The official hierarchy for clearing gate standby upgrades on NW is :

Platinum
Gold
Silver
Platinum Companion
Gold Companion
Other

The official hierarchy for clearing gate standby upgrades on CO is :

Platinum
Platinum Companion
Gold
Gold Companion
Silver
Other

Within each sub-category, you are sorted by the time you were added to the list.