What is the difference between iSpec2200 and S1000D specifications?
I've only worked with S1000D, but what I understand is that they share a common origin in ATA Spec100, with some differences being:

iSpec2200


maintains a focus on "legacy", monolithic publications
scope is more limited to aircraft
specification costs money (https://publications.airlines.org/CommerceProductDetail.aspx?Product=274)


S1000D


focuses on modular data, with concepts such as data modules and the CSDB
scope is more generalized to any product (air, land, sea, etc.)
specification is free (http://public.s1000d.org/Downloads/Pages/S1000DDownloads.aspx)