A Custom..EndCustm block creates a custom toolbar menu. The menu can have up to eight tabs for the F1 .. F8 keys, each with any number of sub-items. The contents of the menu are defined using the Title and Item commands inside the Custom..EndCustm block.
The Title command indicates a new tab, so you can have up to eight of these. After Title, put a string to use as the label for the tab. If the title has no items, you'll be able to select the title itself to insert this label somewhere.
The Item command indicates an item under the most recent tab — you can have as many items under a tab as you like, or none at all. This also takes a string that will be used as the label for the item, and will be inserted any time you select that item from the menu.
Here is an example of the syntax:
:Custom : Title "Animals" : Item "Dog" : Item "Cat" : Title "Rocks" : Title "Plants" : Item "Grass" : ... :EndCustm
In this case, the custom toolbar will display | F1 Animals | F2 Rocks | F3 Plants | … |. Pressing F1 would bring up the Animals menu with items 1:Dog and 2:Cat. Then, pressing 1 or ENTER would insert Dog after the cursor. Pressing F2 would insert Rocks under the cursor, since that tab has no items.
The calculator comes with a default custom menu, which is overwritten whenever you create one of your own. To restore it, select F6 - Clean Up from the home screen toolbar, and select 3:Restore custom default. This also lets you see an example of Custom..EndCustm syntax.
A related command is Toolbar..EndTBar. It also displays a custom menu, but can only be used in a program, and the entries of the menu jump to other parts of the program.
450 - Invalid in a function or current expression happens when Custom..EndCustm is used in a function.
460 - Invalid in Custom..EndCustm block happens when anything other than Title or Item appears in the block.
590 - Invalid syntax block happens when the syntax is somehow wrong, e.g. if there are more than 8 tabs.