For a brief explanation of shidoin and shihan, here are the relevant articles from the Aikikai International Regulations :
Article 15 : QUALIFICATIONS OF INSTRUCTORS
1. With respect to an Aikido organization with Hombu Recognition, the qualifications for instructors are as follows:
In this context, Shihan means "head instructor" Shidoin means "senior instructor" and Fukushidoin means "assistant instructor".
Therefore, each individual organisation
has one Shihan (head instructor), a handful of Shidoin (the more senior ranked yudansha who teach locally and nationally, occasionally internationally) and under each Shidoin a group of Fukushidoin (junior ranked yudansha who teach the regular local classes).
None of these titles are a rank in their own right.
That's the way the UKA seem to understand it anyway!