In version 2.17.3 it's not possible to run mach-composer plan or apply with the flag -s again to run a apply or plan for a specific site. For example when you have 2 sites in your mach config Site A and Site B and you want to run a plan on only Site A by doing mach-composer plan -s A. This should only run mach-composer plan for site A but it will run a mach plan for all sites.