Here are the flags that can be set by the application in the flags field
of the menuitem structure:

CHECKIT
    Set this flag to inform Intuition that this item is a checkmark item
    and should be preceded by a checkmark if the flag CHECKED is set.

CHECKED
    For an item with the CHECKIT flag set, set this bit to specify that
    the checkmark is displayed.  After the menu strip is submitted to
    Intuition, it will maintain the CHECKED bit based on effects from
    other items' mutual exclusions, or, for MENUTOGGLE items, from user
    accesses to this item.

ITEMTEXT
    Set this flag if the representation of the item pointed to by the
    itemfill field and, possibly, by selectfill is text and points to an
    intuitext structure.  clear this bit if the item is graphic and
    points to an image structure.

COMMSEQ
    If this flag is set, this item has an equivalent command key sequence
    set in the command field of the menuitem structure.

MENUTOGGLE
    This flag is used in conjunction with the CHECKIT flag.  If
    MENUTOGGLE is set, a checkmark that is turned on may be turned off by
    selecting the item.  This allows the user to toggle between the
    checked and non-checked states by repeatedly selecting the item.

ITEMENABLED
    This flag describes whether or not this item is currently enabled.
    If an item is not enabled, its image will be ghosted and the user
    will not be able to select it.  If this item has sub-items, all of
    the sub-items are disabled when the item is disabled.

    Set this flag before submitting the menu strip to Intuition.  Once
    the menu strip has been submitted to Intuition, enable or disable
    items by calling onmenu() or offmenu().

HIGHFLAGS
    An item can be highlighted when the user positions the pointer over
    the item.  These bits describe what type of highlighting will be
    used, if any.  One of the following bits must be set, according to
    the type of highlighting desired:

    HIGHCOMP
        This complements all of the bits contained by this item's select
        box.

    HIGHBOX
        This draws a box outside this item's select box.

    HIGHIMAGE
        This displays alternate imagery referenced in SelectFill.  For
        alternate text, make sure that ITEMTEXT is set, and that the
        selectfill field points to an intuitext structure.  for
        alternate image, ITEMTEXT must be cleared, and the SelectFill
        field must point to an image structure.

    HIGHNONE
        This specifies no highlighting.


The following two flags are used by Intuition:

ISDRAWN
    Intuition sets this flag when this item's sub-items are currently
    displayed to the user and clears it when they are not.

HIGHITEM
    Intuition sets this flag when this item is highlighted and clears it
    when the item is not highlighted.