Doclinks is a very usefull feature in Notes and have been a part of the platform since the start, back in 1989, where version 1.0 was released.
Simply put a doclink is a bookmark for a Notes document in a Notes database and by clicking this doclink you open the Notes document it represents. Doclinks can have many uses but I’ll mention two obvious uses, replacement for including attachments in mail and workflow / business logic.
Attachments
Instead of sending an attachment through mail and by that having redundant attachment copies in many mailboxes, you can have a central file container database, from where you can make doclinks to the desired attachment and send the doclink instead.
Workflow
Doclink is very useful in workflows / business logic where changes to documents, approvals and status changes should be addressed by specific people in the organization. The documents that these people have to evaluate would be sent to them as doclinks with description of what action needs to be taken.
There are different ways to make doclinks and send them to people who have interest in them.
Method 1
This is the most common used method when making doclinks. Go to a view in the Notes database from where you would like to make a doclink. Select the desired document or open it. Go to file menu Edit – Copy as – Document link. Open your mail file and compose a message and paste in the doclink.
Method 2
From the same view as in method 1 click on the desired document and drag it to your mail file (needs to be open with an open message). You need to hold the dragged document for a short while until focus is changed to your mail file and while still holding your mouse down for the drag, place the mouse at the position in the message where you would like the doclink and release the mouse.
Method 2.1
Same method as in method 2 but from a open Notes document. Use the same method using the document Tab as source instead of the selected document from the view.
Method 3
Make doclinks of several documents by selecting them from a view using either Shift or Ctrl key. Shift key marks all documents from the first selected to the last selected document while Ctrl key marks one document at a time, anywhere in the view and keeps previous selected documents selected. When the desired documents have been selected, go to file menu Edit – Copy as – table. Open your mail file and compose a message and paste in the doclink table.
Method 4
Programmed as part of a customized application, like workflow and business logic.
Advantage
As a key feature of the Notes platform is offline content and replication, the doclink feature will work, even if you are offline, as long as you have a local replica copy of the database and the replica is up to date.
Other link types
Database links, links to a specific database.
Anchor links, links to a specific anchor element within a document.
View links, links to a specific view in a database.
Introduced: 1.0




In addition to all of the cool features you mentioned they are just another type of hypertext link. One key difference between a URL & other types of hypertext is that they also support a disconnected model. That is, when you are disconnected and accessing data on a local replica, they will point to your local data instead. No error 404 file not found message.
@Ian
Thanks for pointing that out, your are absolutely correct. I have included that point in the post
.