I recall reading somewhere but can't for the life of me remember where that the 'ccmp' and 'aalt' features are treated specially by software and are always processed before other features.
§7.b of the OpenType Feature File Specification seems to confirm this is true for 'aalt', but I can't for the life of me find any discussion of 'ccmp' receiving special treatment. Adobe's discussion in their feature list indicates that "[ccmp] should be processed as the first feature processed, and should be processed only when it is called" but it isn't clear from this whether this means applications are expected to process it first, or that font developers are expected to implement it as the first feature in their feature file. (Also, it's unclear to me what the second part of that description means -- I'd assume that this feature should always be processed, and I've never run across an application which allows you to explicitly select it).
Can anyone clarify this issue for me or provide a link to a source which discusses ccmp in more detail?