The scope of certification defines what your certificate actually covers: which activities, which products or services, and which locations.
It appears on the certificate itself, and it is the first thing a customer checks when they ask for proof of certification. A certificate whose scope does not cover the work you are being contracted for is of no use to them.
Why it drives cost
Certification body audit time is determined by the effective number of personnel within the scope of certification, not by company headcount. A 300-person manufacturer certifying a single 40-person production line is priced as a 40-person organisation.
That makes scope definition the first substantive decision in any certification project, and one with three years of financial consequences: it sets your initial audit days, your surveillance costs in years two and three, and your recertification cost.
Getting it wrong in either direction
Too broad and you pay for audit time you did not need, and you commit to running the management system across activities that never required it.
Too narrow and the certificate does not satisfy the customer who asked for it — which is usually the entire reason you pursued certification. Widening scope later means an extension audit and additional cost.
What cannot be excluded
Scope is about which parts of the organisation are covered. It is different from excluding clauses of the standard. You may exclude requirements that genuinely do not apply — most commonly design and development, clause 8.3, where you manufacture to customer drawings — but only where the exclusion does not affect your ability to deliver conforming product, and the justification must be documented.