BIP39 Import

there must be 12, 16 or 24 words in your mnemonic
we have only the English word list
~~we do not support BIP39 passwords during import~~ (full support added in version 2.0.0)

XPRV Import

we can import a BIP32 HD-wallet root private key in XPRV format
it's assumed to be top-level, and we don't store the parent fingerprint or depth value
SLIP-132 format HD-wallet keys are also supported (xprv/yprv/zprv), but we strip the implied address format

we don't know what day it is, so meta data on files will not have correct date/time
encrypted files produced cannot be changed, and we don't support other tools making them

with Electrum, we support classic payment addresses (p2pkh), Bech32 Segwit and P2SH/Segwit
- however, each wallet must be of a single address type; cannot be mixed (their limitation)
- the same Coldcard could be used in each of the three modes (we don't care about address format)
with Bitcoin Core (version 0.17?), we can do PSBT transactions, which support all address types
we don't support coinbase transactions, so don't mine directly into a Coldcard wallet

we support transactions up to 384k-bytes in size when serialized into PSBT format
bitcoin limits transactions to 100k, but there could be large input transactions inside the PSBT. Reduce this by using segwit signatures and provide only the individual UTXO ("out points").
we can handle transactions with up to 20 inputs to be signed at one time.
a maximum of 250 outputs per transaction is supported (will attempt more if memory allows)

only one signature will be added per input. However, if needed the partly-signed PSBT can be given again, and the "next" leg will be signed.
we do not support PSBT combining or finalizing of transactions involving P2SH signatures (so the combine step must be off-device)
we can sign for P2SH and P2WSH addresses that represent multisig (M of N) but we cannot sign for non-standard scripts because we don't know how to present that to the user for approval.
during USB "show address" for multisig, we limit subkey paths to 16 levels deep (including master fingerprint)
max of 15 co-signers due to 520 byte script limitation in consensus layer with classic P2SH
we have space for up to 8 M-of-3 wallets, or a single M-of-15 wallet. YMMV
only a single multisig wallet can be involved in a PSBT; can't sign inputs from two different multisig wallets at the same time.
we always store xpubs in BIP32 format, although we can read SLIP132 format (Ypub/Zpub/etc)
change outputs (indicated with paths, scripts in output section) must correspond to the active multisig wallet, and cannot be used to describe an unrelated (multisig) wallet.
derivation path for each cosigner must be known and consistent with PSBT
fixed: XFP values (fingerprints) for each of the co-signers must be unique (limitation removed)

we do not support U2F protocol, WebUSB or any other means for random websites to talk to us
only native desktop/mobile apps, or helpers for those, will be able to talk USB to Coldcard

Coldcard will, by default, reject any txn that pays a fee of more than 10% of its total value to miners. This limit is a setting: 10% (default), 25%, 50% or 'no limit'.
Fees over 5% (was 1%) are shown as warnings.

We will hide transaction outputs if they are "change" back into same wallet, however:

PSBT must specify BIP32 path in corresponding output section for us to treat as change
for p2sh-wrapped segwit outputs, redeem script must be provided when needed
any incorrect values here are assumed to be fraud attempts, and are highlighted to user
the redeemScript for p2wsh-p2sh is optional, but if provided must be correct, ie: 0x00 + 0x20 + sha256(witnessScript)
the witnessScript in a p2wsh-p2sh is not optional.
depending on the address type of the output, different values are required in the corresponding output section, as follows
- p2pkh: no redeemScript, no witnessScript
- p2wpkh-p2sh: only redeemScript (which will be: 0x00 + 0x14 + HASH160(key))
- p2wpkh: no redeemScript, no witnessScript
- p2sh: redeemScript that contains the a multisig script, ending in 0xAE
- p2wsh-p2sh: redeemScript (which is: 0x00 + 0x20 + sha256(witnessScript)), and witnessScript (which contains the multisig script)
- p2wsh: only witnessScript (which contains the actual multisig script)