VLAN ID =0用于识别帧优先级这句话如何理解，用在哪里？

ieee802.1q定义的0为协议所预留的值，华三应该是没有vlan0的概念，华为好像是有的

以下是原文： The maximum number of VIDs that can be supported is 4094 rather than 4096, as the VID values 0 and FFF are reserved
以下是对vlan0的描述：虽说是只能表示优先级，但是也没有具体去定义如何使用。可能是想用于发包的时候不想让报文属于某一vlan，但是又想用上vlan的802.1p优先级。

The null VID. Indicates that the tag header contains only priority information; no VID is present in the frame. This VID value shall not be configured as a PVID or a member of a VID Set, or configured in any Filtering Database entry, or used inany Management operation.