Outdoor humiliation is about doing things in public that cause the slave to be embarrassed. There is a wide range of play that can be done with this, and it is very popular in Asian BDSM.
At the mild end, outdoor humiliation could be as simple as a slave going to a restaurant with her Master while the slave is not wearing any panties, or has a dildo in place. Equally, a slave going to dinner with her clearly lesbian Mistress may be sufficient for some people.
For bolder slaves (or more forceful Masters/Mistresses) the slave could be made do things like exposing themselves or going to the toilet while concealed, but while there is the real risk of being seen.
Those slaves who enjoy humiliation can be taken through the city dressed in fetish clothes or obvious bondage.