Planning for single sourcing (insets and variables)

I'm trying to plan for the best way to single source or reuse our content. Most of our content is different but we have login instructions for our applications that are the same for all apps. That's the easy part. We can write those instructions in a separate document and use it when we build the books for each app or use a text inset. But we also have applications where a unique transaction ID is selected when logging in. How do we write common instructions for all these apps where only the transaction ID is different? I was thinking we could use a variable in a text inset, but it seems like each name-value pair is unique and I can't define a list of values for the Trans ID. We would need to maintain many separate variables? We could create an inset leaving the transaction ID blank, then convert it to text and change the transaction ID, but we want to single-source our content in case the login steps change--not just reuse it.