reply to IvanFerrero at https://forum.zettelkasten.de/discussion/1740/anybody-using-this-approach-to-manage-contacts-how#latest
Many of the digital note taking tools that run off of text allow you to add metadata to your basic text files (as YAML headers, inline with a key:: value pair, or via #tags). Many of them have search functionality or use other programmatic means like query blocks, DataView, DataViewJS, etc. for doing queries on your files to get back lists, tables, charts, etc. of the data you're looking for.
The DataView repository has some good examples of how this works with something like Obsidian. Fortunately if you're using simple text files you can usually put them into one or more platforms to get the data and affordances you want out of them individually.
As an example, I have a script block in my daily note in Obsidian for birthdays in my notes that fall on today's date:
```dataview
LIST birthday
FROM "Lists/People"
WHERE birthday.day = date(2023-01-18).day
```
If I put the text birthday:: 1927-12-08 into a note about Niklas Luhmann, his name and birthday would appear in my daily note on his birthday. One can use similar functionality to create tables of books they read with titles, authors, ratings, dates read, etc. or a variety of other data input which parses through your plaintext files. Services like Obsidian, Logseq, et al. are getting better about allowing these types of programmatic searches for users without backgrounds in programming and various communities usually provide help for pre-made little snippets like the one above that one can cut and paste into their notes to get the outputs that they need. Another Obsidian based example that uses text files for tracking academic journal articles can be found at https://nataliekraneiss.com/your-academic-reading-list-in-obsidian/; I'm sure there are similar versions for other text-based platforms.
In pre-digital times, for a manual version of a rolodex like this in paper, one could use different color cards as pseudo-tags (doctors are on yellow cards, family members on blue cards, friends on green cards, etc.) or adding edge notches or even tabs to represent different types of metadata. See for example the edge colored cards in Hawkexpress' Pile of Index Cards: https://www.flickr.com/photos/hawkexpress/albums/72157594200490122