Short answers, pulled from the story.
A phase in combat is a period within a military operation that forms part of a serial chain of logically connected activities, planned to culminate in a defined objective or goal. A phase is usually marked by the achievement of a significant intermediary objective. It may be limited by the time allocated for its execution, or defined solely by achievement of the objective.
At the strategic level, a phase in combat can continue for years. There is no fixed upper limit; objective-bound phases remain open-ended until the goal is judged to have been reached.
Typical phases in offensive operations include preparation phases such as intelligence gathering, operations planning, logistics management, deception and counterintelligence, and assembly. Conduct phases include the initial combat assault and breakthrough. Exploitation phases cover follow-on support, pursuit, objective security, position consolidation, defensive operations, and reorganisation. A stability phase involves enabling civil authorities.
Not all phases of combat include fighting between armed forces. Phases such as intelligence gathering, logistics management, deception planning, and enabling civil authorities involve no direct armed confrontation.
Phases can and usually do overlap, and they can sometimes be planned for parallel execution. Parallel phase execution is often used as part of deception planning to mislead an adversary about where the main effort is occurring.
A lull in combat is typically an unplanned break in operations, arising from the friction of a military campaign rather than from deliberate planning. It differs from a formal phase in that it has no defined objective or deliberate design; it simply occurs when the chain of planned activities is interrupted.