In the Carrier Allocation settings, you can select one of four different rules to define how should the carrier be selected for auto-allocation.
These rules are:

Click on the rule that you want to learn more about to see the relevant article.

This article is about:

This rule allows you to define carrier allocation based on different times. Here you can define different timings and times for which the defined carrier will be assigned. For example, in the 'Timings' drop down you can choose "Order Creation" and add time 9:00 until 13:00. When you then choose a Carrier this will mean that this carrier will be assigned to the orders that are created between 9:00 and 13:00. This allows you to assign different carriers for different times of the day (eg. all orders in the morning go to Carrier A, in the afternoon to Carrier B, and so on).
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Other timings are:
​First Activity From -  First activity means the first milestone (besides Arrive and Depart milestones), and this option can be used for example if there is a scheduled routing stop before collection. 

This means that if the rule is set to this timing, then the carrier will be allocated based on the time of the earliest activity's Schedule From hour. For example, if a routing stop is scheduled from 11:00 to 13:00 and the hours defined in the 'Time' field are 11:30 until 14:00, then the carrier will NOT be assigned to the order. However, if those hours are let's say 10:00 until 11:30 the carrier will be assigned to the order.

First Collect/Deliver From - These timings are the earliest times scheduled for the first Collection and first Delivery milestones (this means that on a route with multiple collection/delivery stops, only the times for the first stops will be taken into account)

For example, when you schedule an order and the collection time is between 14:00 and 17:00, the Carrier will be assigned if 14:00 (Schedule From hour) falls between the Time ranges of the setting (eg 13:00 until 14:30).
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​Last Activity To - Last activity means the last milestone (besides Arrive and Depart milestones), and this option can be used for example if there is still a routing spot after the final delivery. 

The hour that the inserted 'Time' will be checked against is the Schedule To time. If it falls between the specified time range, the carrier will be assigned to this order.
For example, when you schedule an order and the last activity (eg routing) time is between 23:00 and 23:59, the Carrier will be assigned if 23:59 (Schedule To hour) falls between the Time ranges of the setting (eg 23:00 until 01:00).
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​Collect/Deliver To - These timings are the latest times scheduled for the last Collection or Delivery milestones. When you have multiple delivery or collection stops the carrier will be allocated based on the 'Time' range inserted checked against the Schedule TO hour of the last Collection/Delivery milestone.

For example, when you schedule an order and the last delivery time is between 21:00 and 23:00, the Carrier will be assigned if 23:00 (Schedule To hour) falls between the Time ranges of the setting (eg 22:00 until 23:30).

No matter what Timing you choose, you can assign Acceptance Time to the Carrier, which will put a time constraint on how much time the carrier has to accept the order. If the carrier rejects the confirmation or the time expires, the order is assigned to backup carriers.

By clicking 

you can add Backup Carriers, which will be assigned to the order in case the primary carrier rejects confirmation or the acceptance time expires.
For Backup carriers you can also define acceptance time which works the same exact way - if it's rejected or time expires, the order is assigned to another backup carrier, and so on.
In case that there are no more Backup carriers that the order can be assigned to, the system will fire an 'Allocation failed' alert.

Finally, at the bottom of the 'Rules' window, there are three checkboxes, which serve different functions when enabled.

Auto activate TO: When enabled, the TO will be activated automatically upon carrier assignment
​Pending allocation: When enabled, this will move the order status to 'Pending allocation' upon carrier assignment. This means that all carrier-facing e-mails (such as order confirmation) will be blocked until manually released by the Control Tower user.
​Restrict carriers: When enabled, this option allows only the carriers in this allocation method record can be assigned to the order. It means that it will be not possible to manually select a different carrier than one of the ones defined in this rule.