How does an off-field foul play review (the bunker) decide whether a yellow card becomes red?

From World Rugby's guideline on the 20-minute red card in elite rugby:

1. The on-field officials and TMO watch replays on the big screen or sideline monitor.
2. If the act is clearly intentional and highly dangerous after a short number of replays, the referee shows a permanent red card on the field. No off-field review happens.
3. If the officials decide that at least the yellow card threshold is met, the referee may refer the incident for off-field review: the referee shows a yellow card and crosses arms above the head, and the player leaves for 10 minutes (Law 9.29).
4. The FPRO (or the TMO, if there is no FPRO) has up to eight minutes to review all available footage and technology.
5. They advise the referee that the card should either stay yellow or be upgraded to a 20-minute red card. They cannot recommend a permanent red.
6. If upgraded, the referee shows a red card and the arms-crossed signal. The player does not return, and can be replaced after 20 minutes of playing time from the original incident.