Luhmann's numbering and how to know where to add new card

So… all the “instructions” I see advise that if you have new information to add to an existing topic, you “branch” from that card. Fine… how do you know that card even exists? Do you just rely on memory and if you don’t think about it, you just start a new card? Do you search each time?

I’m missing the obvious somehow.

I think of this mostly in trying to use Luhmann’s numbers, but it could apply in general terms with any numbering system if you think of it as “where do I add a link” - Link to old data - fine… how do I know it’s there? I know the modern search tools make this easier than in the manual day, but…

Thanks for any insight!