The Keep button is available in possible options search pop up windows. You see this pop up window when you click on the Possible Entries button in a field on a criteria selection/definition screen.
Identified by pushpin icon –
For example, go to MB51. Press the ‘Multiple selection’ button near Material field
Click on the ‘Possible entries’ or hit F4 key to select few materials.
Now, before double clicking on the material, click the Keep button so that it points downwards (i.e. it is pinned). Now double click.
Try to use it in any lists, e.g. ME21N:
When you are finished, click the Keep button to release the list.
NB: if pushpin is not pulled out before exiting the transaction, the screen can become orphaned, causing an error or a window that cannot be closed.