To Snip an existing Cable in the Design World, first select the cable and then right-click at the desired point along the cable where the snip will be made, open the Cable Context menu and select the Snip here... option.
The Snip Cable dialog opens and lists the selected Cable's Media in a data grid:
Select the associated rows by clicking the checkboxes and confirm by clicking on the OK button at the bottom, which closes the dialog and brings focus back to the Design World Area and the Cable being snipped.
In the above example, the last four fiber strands (fiber 9 - fiber 12) near End A of the Cable (on the left side) are to be snipped.
As a result of a successful Snip operation, a special Label appears at the Snip point displaying the name of the Snip point next to a small scissors icon (Snip 1 in the screenshot below):