The three assignment mechanisms

Every order is ultimately resolved using one — or a combination — of three mechanisms.
Mechanism
When it's used
PG Mode (FIFO)
You have enough own drivers to cover pending orders
Google OR-Tools
There are more pending orders than own drivers, so routes need planning more efficiently
Third-Party Carriers
Your external delivery network
Which of PG Mode or Google OR-Tools is used for a given store is decided automatically, from the ratio of pending orders to available drivers — unless you force one specific mode in the scheduling plan. See Scheduling plans: how the automatic cycle works.
How they combine
Zone configuration decides whether Third-Party gets the first attempt or only acts as a safety net. Everything else works the same either way.
Standard zone — own fleet first:
Own solver (PG Mode or Google OR-Tools).
Idle driver retry — free store drivers the solver didn't use.
Third-Party fallback — last resort.
Third-Party-first zone — carrier first:
Third-Party carrier, one attempt. If assigned, done.
Own solver (PG Mode or Google OR-Tools).
Idle driver retry.
No further fallback — the order already had its Third-Party turn.
The rule that keeps it fair
Each order gets exactly one attempt with each resource type — own fleet and Third-Party — per assignment cycle.
The zone configuration only decides which one goes first, never how many attempts an order gets:
Zone type
Attempt order
Total attempts
Standard
Own fleet → Third-Party
2
Third-Party-first
Third-Party → Own fleet
2
Always two. Never three, never one.
Before the order is left for the next cycle
There is one more relaxed attempt within the same cycle — see Last-Option: the final safety net.
If the order is still unassigned after that, the next scheduled run starts completely fresh, including a brand-new Third-Party attempt. How soon that happens is set by the store's scheduling plan.