Word for the aspect of an object which makes it different from a group of otherwise identical objects

Say I have these three statements:

1. "John likes pasta."
2. "Mary likes pasta."
3. "Joseph likes pasta."

For these three sentences with the format "[name] likes pasta", I’m looking for a word for the "[name]" with respect to the fact that this particular part of the sentence is the only thing which makes it different from the other two.

The usage I would imagine is this:

The word of sentence 1 is "John".

For context, the word I am looking for is to be used as a parameter name for a function which takes a string and embeds it into a template string, similar to the following:

function embedInSentence(word_here) = word_here + " likes pasta."

I remember a book I had about cars when I was younger described "exterior identification points" of different models, which could be used to tell that particular one from another model/trim level. It’s the closest thing I have to a fitting word or phrase, but it’s quite long and doesn’t fit my use case – I’m looking for a more abstract term, such as "identification point" but as a single word.