Every answer says what moment of the market it describes, what spot it was priced against and which expirations it covers.
The envelope
Every response is {"data": ..., "meta": ...}. The meta block is the same on every endpoint.
meta.as_of, as_of_ms
The moment of the market data in this answer. Age counts from here.
meta.computed_at
When this answer was computed.
meta.spot.price
Spot for this answer.
meta.expirations
Mode (nearest or term), count and dates.
meta.stale
True when this answer couldn't be refreshed just now.
Units
Money
USD, never scaled. net_gex: 520000000 is 520 million dollars.
Gamma exposure
USD per 1% move in spot, from the dealer's side: calls positive, puts negative.
Delta exposure
Shares.
Vanna exposure
Shares per 1 vol point.
Charm exposure
Shares per day.
Prices
USD per share. An option price of 1.05 is $105 per contract.
Premium
USD: size × price × 100.
Quantities
Contracts.
Volatility and ratios
Fractions: 0.18 is 18%. There are no percent fields.
Times
RFC 3339 in UTC. Dates are YYYY-MM-DD. Durations carry their unit in the field's name.
Conventions
Expirations
expirations=N takes the nearest N, 1 to 50, default 4. expirations=term takes the listed expiration nearest each of 5, 7, 14, 30, 60, 90, 120, 180, 270 and 365 days out.
Symbols
Use the familiar ticker: SPY, SPX, NDX, AAPL. Weekly index roots are covered under SPX and NDX.
Ranges
Endpoints that answer rows per session take from and to, inclusive, up to 31 sessions a call. Today is never part of a range.
Caching
A finished session's answer carries an ETag. Send it back as If-None-Match and an unchanged answer is 304 Not Modified.
A session still loading
A past session that isn't ready answers 202 with Retry-After. Ask again after that many seconds and you get the usual 200.