Where a ΔT came from: one of the model's segments above; "pinned", a
caller's pin; or, on the IERS basis ("iers-utc/1", docs/time.md), the source
of its UT1 − UTC, "observed" or "predicted". "fallback" is the IERS basis
with UT1 − UTC taken as 0 ± 0.9 s past the table's ends, which rc.15's first
cut recorded and receipts from it carry; later releases do not give it, and
keep it so those records stay readable.
Where a ΔT came from: one of the model's segments above; "pinned", a caller's pin; or, on the IERS basis ("iers-utc/1", docs/time.md), the source of its UT1 − UTC, "observed" or "predicted". "fallback" is the IERS basis with UT1 − UTC taken as 0 ± 0.9 s past the table's ends, which rc.15's first cut recorded and receipts from it carry; later releases do not give it, and keep it so those records stay readable.