I did a post a while back on setting up the signing limits. These are what you can then use in your workflow to identify who has the authority to approve something. The basic setup you will likely need is to setup limits by Job types and job types are setup in the HR module. Even if you are not using HR in AX then you will need to setup a minim level of employees, positions, jobs to match what you need for your workflows.
Here is a quick overview.