Historically the most popular SIA licence held is the Door Supervisor. This is because, as well as being able to work in pubs and clubs, it also allows you to undertake all of the activities covered by an SIA Security licence.
Additionally, the Door Supervisor Course covers working in Event Security (e.g. at the large music festivals that run throughout the summer months). This means that one SIA licence enables you to work in three sectors of the industry, thus maximising your job opportunities.