The definition of Serving Cell would be very confusing in many cases. The formal definition of the Serving cell from 36.331 is as follows.
Serving Cell: For a UE in RRC_CONNECTED not configured with CA there is only one serving cell comprising of the primary cell. For a UE in RRC_CONNECTED configured with CA the term 'serving cells' is used to denote the set of one or more cells comprising of the primary cell and all secondary cells.
The most confusing parts would be how to define the Serving cell in CA (Carrier Aggregation) situation. With this simple definition of the Serving Cell, refer to following pages to see in which context the definition of the serving cell matters and in what way it would confuse you.