From Ubiquiti Wiki

In order for AirMax to classify and differentiate types of traffic when applying QoS rules, the traffic must have a special value set in the IP Header DSCP field. The originating software or hardware device is responsible for applying this value, and only if this field is set will traffic be prioritized.

There are four traffic categories: Best Effort, Background, Video, and Voice, which range from priorities lowest to highest in that order.

By default, all traffic is classified as "Best Effort", meaning there is no prioritization applied. The other categories can be defined with the following values:

Background:
(Format ToS Decimal : ToS Hex | DSCP Decimal : DSCP Hex)

• 0  : 0x00 | 0 : 0x00
• 32  : 0x20 | 8 : 0x08

Video:

• 40 : 0x28 | 10 : 0x0A
• 160 : 0xa0 | 40 : 0x28

Voice:

• 48  : 0x30 | 12 : 0x0C
• 136 : 0x88 | 34 : 0x22
• 184 : 0xb8 | 46 : 0x2E
• 224 : 0xe0 | 56 : 0x38