I have a question related to Use Case diagram for a mobile app and charging.
So let's say My use case is Set Charging mode. Charging mode can either be Immediately or Scheduled Later. What Relationship should be between the Set Charging Mode UC and the two different options?
I have thought about using Generalization but I am not 100% sure this is correct.
Not knowing much about your context, I nevertheless understand that "Set charging mode" is not really a user goal: it's not a set of behaviors with observable results of value for the user. As such it should not be a use case.
In principle, "Set charging mode" would be part of the description of the real use case (e.g. getting something for which the user will be charged). Or maybe you'll specify the expected behaviors in a separate activity diagram.
If I'd be wrong, or if despite the advise you'd opt for a use case, the use case should require a choice, and a mutually exclusive one. This would be part of description of the use case Set charging mode, or indicated as a comment for the use case.
If I'd still be wrong and if you'd really really want to show two use cases, you could indeed either have two specific extensions to the choice or consider specialization. However, from your description, I still understand that these use cases would not be independent (Set charging mode or Set charging mode to later make no sense on their own). Moreover they seem far too detailed for a use-case. I'd therefore really advise not to follow this way keep UC for the big picture and avoid getting lost in details better documented elsewhere.
To complete Christophe's answer, you have to keep in mind 2 things (at least):
