KB: Add a substitute component in ActiveBOM

Solution Details

To add a new BomDoc to the project, use File ► New ► ActiveBOM or right-click on the project in the Projects panel to select Add New to Project ► ActiveBOM. Note that each PCB project can only include one BomDoc.
 

Substitute with a part identical in specs:

A substitute component can be specified as Part Choice solution if it is identical in technical specs but procured differently (supplier, packaging, etc.)  Right-click on the component to be substituted, and select 'Add Solution' to specify Part Choice solution.
https://www.altium.com/documentation/altium-designer/managing-activebom-solutions#!creating-solutions
Multiple solutions can be prioritized manually (Primary, Secondary1, Secondary 2...) with User Rank, by clicking on stars.
https://www.altium.com/documentation/altium-designer/managing-activebom-solutions#!ranking-the-solutions
 

Substitute with a part equivalent functionally:

The substitute can be specified as Alternate Part, if it varies in specs (voltage rating, tolerance, etc.) but equivalent functionally.  Right-click on the component to be substituted, and select 'Operation » Add Alternative Item' to specify Alternative Part.
https://www.altium.com/documentation/altium-designer/managing-activebom-solutions#!including-an-alternative-part
The option is available only for components managed in a workspace.  For unmanaged components from file-based libraries, it may be expedient simply to use Part Choice solution in this case as well, granted the component is not "managed" strictly to begin with.


The Component Search dialog will open, where you can search the connected Managed Content Server for a suitable alternative. 

An Altium 365 Pro subscription Includes the IHS Markit to compliment the supply chain intelligence from Octopart.  IHS augments component lifecycle and alternative part data to deliver unparalleled supply chain information and insights.  https://www.altium.com/part-data-sources (built into MPS panel)

This video from Altium Academy may help if you are a Pro level subscriber:
https://www.youtube.com/watch?v=nZmBD9VXhTA