Formally there isn't. The subcommand argument is a required positional argument, where the 'choices' are the subparser names (and their aliases).
That's evident in the help message, were {cmd1,cmd} are shown as choices.
usage: ipython3 [-h] {cmd1,cmd2} ... positional arguments: {cmd1,cmd2} optional arguments: -h, --help show this help message and exit
The latest Python has a 'bug' that actually lets it be optional. In other words, it does not raise an error if you don't give any positionals. It's a bug because it changes previous behavior, and most people what it to be required. But even when it is optional, and cmd1 defined as the default, it won't run the cmd1 parser on the remaining arguments. And any 'positional' argument will be treated as a wrong command string.
I think the best you can do is define one or more positionals. One might have choices and default, and be optional (nargs='?'). The rest (or other) might has nargs='+'. By playing around with those options you could approximate the desired behavior, but it won't make use of the subparsers mechanism.
Another way to think about the issue, is to consider an input like
prog one two
Should it interpret that as prog src dest or prog cmd id. The only basis for saying it should be the former is the fact that one is not one of copy or resume. But it isn't that smart when it comes to handling 'choices'. It assigns a value based on position, and then tests whether it meets criteria like 'choices' or 'type' (ie. integer, float v string).
