The horizontal bar of the T-shaped professional describes what?

The T-shaped professional model is used in business analysis to describe the ideal mix of skills and knowledge. The 'T' shape consists of a vertical bar and a horizontal bar. The vertical bar represents the deep and specific domain knowledge or core expertise (e.g., business analysis techniques, specific industry knowledge). The horizontal bar (the top of the 'T') represents the multi-disciplinary breadth of skill and knowledge that an individual can apply to various situations. This breadth includes soft skills like communication, facilitation, leadership, and a broad understanding of different business areas (like finance, operations, IT, and strategy). It is this breadth that allows the business analyst to collaborate effectively across different teams and disciplines, understand the context of the change, and recommend truly holistic solutions.

(Reference: BCS Foundation Certificate in Business Analysis / BCS Business Analysis Practice – Competencies of a Business Analyst)