When editing a computer record the user may assign multiple Operating System to a computer record. This is done from edit mode of a specific computer record by clicking the 'Edit' icon displayed in the 'Installed Operating Systems' subsection. (This icon displays in the 'Installed Operating System' title, and only when editing an existing computer).

This will generate a pop-up window which will allow to manage the OS assigned to the current computer record. The pop-up window will show a list of OS currently assigned to the computer. This popup uses the standard toolbar:

Note, that there is another way to assign an OS to a computer - by editing a specific OS record, and changing the assignment from there.