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Q: in 5GS, can a GBR flow be a default QoS flow?

A: Yes, but it is not recommended. When EPS interworking is required, this PDU session is unlikely to be transferred to EPS successfully. In EPS, a GBR bearer must be a dedicated bearer. A dedicated bearer must be linked to a default bearer. Consequently, the transfer is not supported.

Reference
Subclause 5.7.1.1 in 3GPP TS 23.401,
The QoS Flow associated with the default QoS rule provides the UE with connectivity throughout the lifetime of the PDU Session. Possible interworking with EPS motivates the recommendation for this QoS Flow to be of type Non-GBR.

Subclause 5.7.2.7 in 3GPP TS 23.401,
Interworking with EPS is not possible for a PDU Session with a GBR QoS Flow as the QoS Flow that is associated with the default QoS rule.