The difference between keynoting and tagging a material is the information that is displayed in the tag and which tag family
Revit uses.
Revit has separate categories for keynotes and material tags.
Using keynotes
When you use a material keynote,
Revit looks for a tag family assigned to the keynote category. If the tag is not loaded in the project, you are prompted to load one.
The keynote family displays one or both of the values available from the keynote table as assigned to the material's keynote parameter. If the material is not assigned a keynote, you will be prompted to select a keynote value when the material keynote is placed.
To place a material keynote, click Annotate tabTag panelKeynote drop-down (Material Keynote).
Using tags
When you use a material tag,
Revit uses a tag family assigned to the material category.
The default tag displays the value stored for the material's Description parameter, if it is assigned.
To place a material tag, click Annotate tabTag panel (Material Tag).