A movement joint is an intentional separation that accommodates movement between building components. Its function and construction depend on materials, structural behaviour and adjoining elements. A visible line does not automatically represent a fully developed movement detail.
How this term relates to your project
The original house and extension may move differently. Ask where that behaviour is accommodated and how finishes cross the junction. A continuous floor or wall covering may need different treatment from separate surfaces. Coordinate the structural decision with waterproofing, airtightness and insulation wherever those functions meet.
The detail should identify interrupted layers and explain how other functions continue. Do not later cover a designed joint solely for appearance without checking its purpose. Raise aesthetic preferences early so technical behaviour and finish are addressed together. Keep the agreed detail with the project records for future repairs or changes to the surface.