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

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

This rule allows you to choose the allocation percentage of the total orders, per carrier. You should define the percentages so that the sum from all carriers will equal 100% (eg Carrier 1 - 30%, Carrier 2 - 50%, Carrier 3 - 20%). Following these percentages the orders will be split accordingly meaning that out of 10 orders, Carrier 1 would be allocated 3 orders, Carrier 2 - 5 orders, and Carrier 3 - 2 orders.
For the 11th order, the allocation would choose the carrier based on how high the
allocation percentage is - in this case, Carrier 2.

In the Accept. Time field you can define how much time does a carrier has to accept the order (if confirmation required). In case the Carrier rejects confirmation or time expires, the next Carrier will be assigned based on the allocation percentage.
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If you define the Minimum and Maximum Orders fields, then the system will prioritize trying to fill the minimum values first and then follow the same logic as described above - assign the next carrier based on how high the allocation percentage is.
Once the Maximum Orders limit has been reached for a Carrier, they will not be assigned any more orders, and the assignment will pass onto the next carrier for which the Maximum Orders limit has not been reached (if exists). 

Once the limits of all the carriers are reached, then the orders will be assigned to the Backup Carriers. If none are selected, then the system will continue assigning the orders based on percentage allocation.

By clicking 

you can add Backup Carriers, which will be assigned to the order when the carriers have reached their Maximum Order limit, or the available carriers all rejected the confirmation/confirmation time expired.
For Backup carrier, you can also define acceptance time which works the same way as for the carrier - 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 go back to the Allocation Percentage logic, disregarding the Maximum Orders limit.

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 to be assigned to the order. It means that it will not be possible to manually select a different carrier than one of those that are defined in this rule.