In APS, function and purpose are related but distinct.
Function concerns the contribution a particular structure or process makes to maintaining the organisation of a living system. Purpose concerns the organisation of the system’s activity as a whole relative to viability.
A heart has the function of circulating blood. The organism’s purpose is not “to circulate blood,” but to sustain its continued persistence through coordinated viability-oriented activity.
Function therefore refers to local contribution within organised systems, whereas purpose refers to the global organisation of activity relative to persistence.