You can document which metadata components are used to support which part of your business process or architecture by searching and linking those components directly onto diagram boxes.
Prerequisites
Synced Salesforce Org Model
Edit rights in the Org Model
Editor rights on a Diagram
Search for metadata
Switch to edit mode on the diagram. Click on an activity box. A floating toolbar will appear. Select the 'Add documentation' icon. From the dropdown select 'Salesforce metadata'.
You will see a modal window. Pick one of the synced Org models and pick the metadata type (e.g. field, object, etc.).
Start typing letters from the label or API name of the component you seek.
By default, all search is done using "Starts with" query. So if you type in "Account", we will bring back metadata components which have names starting with "Account" but not if they have that word later in their name.
You can still perform "Contains" query if you prefix the string you are searching for with "*", e.g. "*Account".
Within seconds the system will bring back a list of candidates that you can scroll through. Clicking on a search result will load the required component.
Exploring metadata to diagram links
Linked metadata components will be visible under the little cloud icon. Click on it to see connected metadata components.
On the metadata component, you can find linked diagrams in the right panel in the documentation tab.