Units

One tracking costs 1 unit. The number you can still create is your balance, plus credit, minus units already held by creates that are still processing.

Create a tracking reserves a unit only for an item that is queued. If the carrier is unknown and you did not send carrierCode, that item is not queued and does not reserve a unit.
Create trackings (async) reserves one unit for every item in the request, before processing finishes.
If you do not have enough, the whole call returns 400 and nothing is queued. Two calls sent at the same time can both pass. The later call adds to the units already held, and the next call is rejected.
The tracking API cannot return balance or usage. Sign in to the Portal and open Billing.

The numbers

balanceaccount

Units on the account.

creditaccount

Extra units added to the balance.

pendingaccount

Units held by creates that are still processing. Released when processing finishes.

messagestring

The only field on a 400. Example: Insufficient units: has 2, requested 10, pending 1.

`v1`	Create	One unit for each item that is queued. An unknown carrier with no carrierCode is not queued and does not reserve a unit.
`v2`	Async create	One unit for each item in the request, reserved before processing finishes.