Which field in the IPv6 header is used for QoS. or specifying the priority of the packet?

The Hop Limit field is an 8-bit field that limits the number of hops a packet can traverse before being discarded, equivalent to IPv4 TTL - it has no QoS function.

The Traffic Class field is an 8-bit field in the IPv6 fixed header that allows originating nodes and forwarding routers to identify and differentiate traffic classes and priorities, directly enabling QoS mechanisms. The upper 6 bits are used for Differentiated Services Code Point (DSCP) marking, and the lower 2 bits are used for Explicit Congestion Notification (ECN). This makes it the functional equivalent of the IPv4 Type of Service (ToS) field.

The Version field is a 4-bit field that simply identifies the IP version as 6 and plays no role in QoS or traffic prioritization.

The Next Header field identifies the type of header immediately following the IPv6 header (such as TCP, UDP, or an extension header) and has no QoS or priority function.