Is vkCmdExecuteCommands "dynamic"?

Is vkCmdExecuteCommands() a way to specify a command buffer that can be dynamically modified, or is it just a way of adding the contents of one command buffer to another?

In other words:
Record secondary command buffer A
Record primary command buffer B, including vkCmdExecuteCommands to execute A.
Modify A.
Execute B, and the modified contents of A are executed.

The later: