This is something that I'm very interested in - the ability to "categorize" a note with a defined field (used to hold undefined contents). As Eldorado pointed out, when I select the "Info" icon at the top right in a Snippet view of a Note, OR when I select List view (top or side), the note's system fields are displayed. These fields include things such as: Notebook, Created, Author, Sync Status, etc. I can select and edit some of those fields (date edited, author), but not all all (subject) - which is very surprising, since I wouldn't think you actually want to edit the "date edited" field (as a system property), but likely WOULD want to edit "subject".
I would like to create and edit custom fields.
I've been using Tags, however, there are many types of data for which I do NOT want to create a unique tag. For example, I'm building a reading list system, using web clipper to collect information about various books to read (or have read), from Amazon, Gutenberg, etc. Each title is a unique note. While I can see the sense of using tags for genre, or the read / to read status, it doesn't make sense to use tags for things like author name, or reading list priority number; both of which would quickly generate far too many unique tags. For example, a "Great Books" reading list covers well over 70 books; simply ranking them, or subdividing into genres, or time periods, or some other subcategorization to then rank, would result in LOTS of tags.
I currently have a GTD system managed in EN, in which I track project activity by month; so just for 2019, I've got 12 subtags (201901, 201902, etc.)
I would find a capability to create and manage custom fields (or "properties") VERY useful and efficient. Is there currently such a way to do this? Or is there a similar request I can upvote?
FYI, I use EN on a Windows system, as well as on my Android phone and tablet.
TIA
Ed