Capacity allocation in SAFe is the explicit allocation of a portion of backlog capacity to Enablers, which include activities like building or maintaining the Architectural Runway. This ensures that teams continuously invest in infrastructure and architectural work needed for sustainable development, rather than focusing solely on immediate feature delivery.
"Capacity allocation ensures that a portion of team and ART resources is always dedicated to Enabler work, such as building the Architectural Runway, allowing the enterprise to invest in future capabilities while delivering current value." (Source: SAFe 6.0 Framework: Capacity Allocation; SPC 6.0 Guide "Building Solutions with Agile Product Delivery")