From the keycloak documentation and https://keycloak.github.io/docs/userguide/keycloak-server/html/auth_spi.html

it is not very clear to me what the OPTIONAL setting for an execution mean.

For example, when we have the following:

Forms Subflow - ALTERNATIVE
           Username/Password Form - REQUIRED
           OTP Password Form - OPTIONAL

When can it enter the Optional OTP form? Do we need to add some code (some condition ?) in the UsernamePasswordAuthentication Code, so it enters the optional OTP form authenticator? Or something else? I am not so clear about the concept of this optional field and how to enter it. Can someone please explain this in detail?