Auxiliary Board members are appointed by the Continental Counsellor for their region and work at national and local level. They serve on one of two boards, and the division between them is not administrative convenience but a statement about what a religious community needs.
The Board for Propagation is concerned with teaching and with growth: the expansion of communities, the training of believers, the activities through which the Faith is presented to those outside it. The Board for Protection is concerned with the community's internal cohesion, with its resilience against opposition from outside, and with the handling of internal dissent that the Faith regards as threatening its unity.
Pairing the two is deliberate. A religion that only expands and never attends to its own coherence, or that only defends itself and never grows, is understood in Baha'i writing to be failing in one direction or the other, and the appointed arm carries both concerns in parallel rather than assigning them to the same people.
Members may in turn appoint assistants to help with a particular area of work, which adds a further layer beneath them and pushes the appointed arm down to the level of individual localities. Peter Smith's reference work on the Faith sets out the structure and the sequence in which it was built.