Modify > Sub-Mesh Shells subdivides the selected shell(s) by using the user-specified number of segments along both edges of the shell(s).
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Remember that node numbers can be turned on for reference by clicking View > Display Options > Shell Display Options > Shell Number, or by using the button on the Quick Access Toolbar.
To relate node numbers to the reference corners 1 through 4 of a selected shell, click Tables > Shells.