A specific compound, fuse, link, hitch etc. synonym needed

I am going through a couple of thesauruses (thesaura?) looking at alternatives for words like ‘link’, ‘join’, ‘hitch’ to find an apropriate one for my abstract context. My problem is that 1) as always, these dictionaries are quite unhelpful in picking up the differences, and 2) as a non-native speaker, I encountered some of these words only in particular contexts, while they may be much more generic, meaning my mental association can be flawed.

Basically, what I have are ‘extensions’ or links, chaining things together, which exhibit the following properties:

1. Each link adds a single unit;
2. Both the nature of the connections and the chained units are important;
3. The whole compound is an entity which exists in itself, with a separate purpose, not as a collection of its components, so words like ‘listing’, ‘point’ are out;
4. It is not homonogenous, each ‘link’ is a distinct, recognisable section of the whole, so words like ‘blend’ are out.
5. There is no specific shape, or puzzle-like nature required of the connections;

Additional restrictions:

1. Words ‘join’ and ‘catenate’ are reserved;
2. Word ‘link’ (and probably a couple others) are too generic and thus sadly out.

To confound things more, I have two very distinct types of this kind of thing I need separate words for:

1. An extension-like thing, which is like adding another bullet point to a list: what came before is still a part of the compound, and the added unit is in no way special among those before it;
2. An annex/appendix-like thing, where what came before is ‘closed off’ and the unit added is clearly distinct from the previous ones. The word ‘annex’ actually isn’t bad, but is one of those that seem to be used only in certain contexts and I fear might sound strange.

They both use essentially the same mechanism and the difference is largely in interpretation/use and not their core nature. I hope I did not overdo it in an attempt to help highlight the specifics?