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The --extend option is define within catkin which generates the setup files (not catkin_tools).

The chaining of workspaces basically works as follows:

• If you build a workspace A without sourcing any setup file before the workspace A is not chained (so it can't use anything from other workspaces)
• When you source the setup file of workspace A and then build a workspace B that workspace B will overlay the sourced workspace A).
• When you then source the setup file of workspace B and then build a workspace C that workspace C will overlay the sourced workspace B and the workspace A).

What you can commonly not do is source two setup files. At least you won't get the result you might expect. Lets consider the following example:

• Workspace A was built without sourcing anything else before.
• Workspace B was built without sourcing anything else before.
• When sourcing the setup file of workspace A and then of workspace B the resulting environment will contain only the stuff from B. The reason is that the setup files will try their best to undo the environment changes from workspace A when sourcing the setup file of workspace B.

• Their are two options to get a "merged" environment:

  • Rebuild one of the workspaces with the other workspace sourced to chain them.
  • Source the second setup file with the option --extend.

After describing all this for ROS 1 finally getting to the difference in ROS 2:

The setup files in ROS 2 / ament do not contain any logic to undo changes of previously sourced workspaces. Simply because that logic is very fragile and likely to be incomplete in non-trivial cases. As such they also don't provide the --extend logic since you can simply source two setup files from independent workspaces and the result will be an environment containing both.