As an aid for myself, I made a summary table:
| Type | Antiform | Usage | Stable? |
|---|---|---|---|
word! |
LOGIC! | Just OKAY (truthy) or NULL (the only falsey state) |  |
block! |
SPLICE! | Values without surrounding container (empty is NONE) | |
fence! |
DATATYPE! | Description of a value's type | |
blank! |
VOID! | Panics on WORD! access, multi-step evaluations discard |  |
frame! |
ACTION! | Trigger function execution from words | |
tag! |
TRASH! | Panics on WORD! access, console does not display | |
group! |
PACK! | Multi-returns from a function (empty is "HEAVY VOID") | |
error! |
FAILURE! | "Hot" failure signal, elevated to panic if not triaged | |