In trying to design an error-correction mechanism for self-assembling systems, I have "invented" a combinatorial object that seems natural enough that it must have appeared in the literature somewhere before.  However, I don't know the keywords to search on to find it.  So I'm hoping someone here can point me in the right direction.

An example on a five-letter alphabet is this:

The set of four-letter words, where each of the four letters is chosen from the five-letter alphabet.  The permutations are ordered as the first one, the second one, etc.  For letters $a,b$ in the alphabet, once the substring $a -^i b$ appears, it can never appear again, where $-$ is a wildcard for any letter, and $i \geq 0$ (so $-^0$ is the empty string).

I'm interested if we allow letters to appear multiple times in a word, if we require each letter appear at most once, and both in results that are existential, and also algorithmic (finding lists of such words), and other properties.

> What is the name of this and/or related objects?  What is a standard and/or state-of-the-art reference?

Thanks very much.