Function-as-a-Service (FaaS) is a Serverless Cloud paradigm where a platform manages the execution scheduling (e.g., resource allocation, runtime environments) of stateless functions. Recent developments demonstrate the benefits of using domain-specific languages to express per-function scheduling policies, e.g., enforcing the allocation of functions on nodes that enjoy low data-access latencies thanks to proximity and connection pooling. In this paper, we consider APP, one of the languages proposed to specify Allocation Priority Policies in FaaS implemented on top of the popular OpenWhisk serverless platform. The aim of our operational semantics is twofold: on the one hand, it represents the underlying substrate necessary for the application of formal analysis techniques, on the other hand, it can drive consistent implementations of APP on top of the numerous serverless platforms recently proposed.