The amount of + Tier should have the limit of the integer of the signals, as there's a mod that add unlimited tiers. (https://mods.factorio.com/mod/infinite- ... rom=search) Signal to switch automatically to another Tier (Speculation)

Example:
+1 Tier:
Left-click: Common -> Uncommon -> Rare -> ...
Right-click: Legendary -> Epic -> Rare
+2 Tier:
Left-click: Common -> Rare -> Legendary
Right-click: Legendary -> Rare -> Common
3+ Tier:
Left-click: Common -> Epic -> Legendary (since there's no higher than Legendary)
Right-click: Legendary -> Uncommon -> Common (since there's no lower than Common)
The idea to make upgrading with blueprints easier while being flexible with mods that add more tiers.