If you have checked the setting Allow revision changes during order registration?, under the heading Orders that you will find under the Manufacturing tab in the Settings procedure, the Revision field appears allowing you to enter another revision number than the active revision (which will be used by default if not).
If you use alternate preparations, the Revision field will be visible from the beginning (without having to check the setting). Then you can also enter another revision than the active revision, which will be used by default if not.
If you are simulating a structure order (incorporated parts at several levels), the latest revision for the incorporated parts will be used, regardless of which revision you have selected for the main part.
If you have set a term for revision for the part in the preparation, it will be taken into account here. Read more about how terms and conditions affect manufacturing orders under Alternate Preparations and Loading Simulation.