The markCharacter is a property of menu items that specifies whether a “check mark” or other marking character should be shown in the empty margin to the left of a menu item’s name to indicate that the item currently applies or is active.
Currently, Stacksmith provides two constants to specify a standard marker: checkMark shows a check mark next to an item, indicating that it is “on”. Specify mixedMark to show a little dash next to the item instead. This is usually used to indicate the item is partially “on”, e.g. if you have multiple items selected, and the menu item is only “on” for some of them (e.g. in a text field, if you select a word that starts with a bold character, but continues in plain text style, the “Bold” and “Plain” menu items would both be marked with a mixedMark). Specify empty or "" to remove any mark characters from the menu item.
From → Menu Item Properties