A subphase is a part of a larger phase.
A subphase is like a small part of a bigger thing, like a piece of a game or a part of a story.
Examples:
The morning is a subphase of the day.
Eating breakfast is a subphase of the morning.
The subphase of the game is very fun.
The subphase of the story is the best part!
In the movie, the subphase with the dragons is the most exciting.
The subphase of the party with the cake is the best part.