A Collapsed Sub-Process may be attached to a process level, consisting of one or more Visio sheets. This relation allows a back and forth navigation between the collapsed sub-process element and its process level. This feature needs an already created process level in order to be selectable. Once you selected the dialog Attach Process Level… in the Context menu the following dialog should be seen:

Already attached process levels are presented in italics and cannot be selected. The Process level which harbours the Subprocess is already selected, where as the other ones are not. Keep in mind that only Process Levels in THIS document will be available for selection. For Document external Process Levels (e.g in the Repository) you need to use the Calling activity (process, collapsed). You can select a desired process level by left click which marks it by a blue background (see Picture). To confirm the attachment hit OK. 

Expand a Subprocess to a New Process Level

If you want to create a new Process Level to attach to the selected Sub Process you can use the Expand to new process Level option in the context menu. This option will open the following window:

I this window you have to input the name for the new, independant process level.  Furhter you can specify if the connected Pools and the parent lane are reused. This means the parent lane will be automatically generated in the new Process Level and it further will be attached to the original lane. In terms of BPMN this means that the lanes are the same object and you will not get into trouble with your process landscape. By reuse connected pools external pools which are connected to the process Level by any means ( Message Flow /Sequence Flow) will be copied to the new process level. By hitting OK a new Sheet with an independet Process Level will be created.

Subprocesses with attached Process Levels

Once you have attached a process Level to a subprocess your mouse icon will change upon hovering over the subprocess. This signals that the subprocess is attached to another process level. You can jump directly to the attached process level by pressing CTRL + Left Mouse Button on the subprocess. You can navigate back to the main process level via a right click on the sheet and then choose Collapse in the Context Menu.

This way you are able to navigate efficently through your process levels.