The Traffic Class in every IPv6 packet is a byte also known as the Differentiated Services field. It is treated in every respect exactly like the same field in every IPv4 packet (originally named the TOS octet in RFC791). It contains six bits of differentiated services code point followed by two ECN (Explicit Congestion Notification) bits. RFC8100 gives a good overview of current differentiated service interconnection practices for ISPs. RFC5127, RFC4594, RFC5865, RFC8622 and RFC8837 also describe current practice.
ECN is intended for use by transport protocols to support congestion control.
The Flow Label is a 20 bit field in every IPv6 packet, although as it name indicates it is only relevant to sustained traffic flows. The sender of a packet should fill it with a pseudo-random non-zero value unique to a given traffic flow, such as a given TCP connection. It can then be used downstream in support of load balancing. By definition, the 20 bits have no semantics, although some deployments are known to have broken this guideline, which would interfere with load balancing. See IPv6 Flow Label Specification, Using the IPv6 Flow Label for Equal Cost Multipath Routing and Link Aggregation in Tunnels and Using the IPv6 Flow Label for Load Balancing in Server Farms.