Are there any Tag feature in DirOpus?

wondering if there are any tags features

There are several. What kind of tags are you looking for?

Most (e.g. metadata fields for music files, and generic tags and comments/descriptions) can be accessed via the metadata pane (Lister > Metadata Pane, or the toolbar button for it, or F9 hotkey).

There are also labels (usually used to color files/folders).

I want to tag some important documents and folders so it would be easy for me to search / list them later when I need. More of easy accessibility.

Using labels probably makes the most sense for that:!Documents/Labels1.htm

Note that if the labels are stored in NTFS, they can be lost when editing files with some software, if the software deletes the old file and creates a new one in its place without transfering the old metadata over.

You can store labels in the config file instead, if that's a problem with the software you're using, although that will also mean the labels don't move with files/folders when you move or rename them.

You can search for files with labels using Tools > Find Files > Advanced.

Thank you. Will explore more on this.