(Optional) Who owns the corresponding NFToken. If the offer is to buy a token, this field must be present and it must be different than the Account field (since an offer to buy a token one already holds is meaningless). If the offer is to sell a token, this field must not be present, as the owner is, implicitly, the same as the Account (since an offer to sell a token one doesn't already hold is meaningless).
Identifies the NFToken object that the offer references.
Indicates the amount expected or offered for the corresponding NFToken. The amount must be non-zero, except where this is an offer to sell and the asset is XRP; then, it is legal to specify an amount of zero, which means that the current owner of the token is giving it away, gratis, either to anyone at all, or to the account identified by the Destination field.
The Amount field is not valid. For example, the amount was zero for a buy offer, or the amount is denominated in fungible tokens but the NFToken has the lsfOnlyXRP flag enabled.
The specified Expiration time is invalid (for example, 0).
The sender already owns too many objects in the ledger, or there are already too many offers to buy or sell this token.
The specified Expiration time has already passed.
The Amount is denominated in fungible tokens, but one of the trust lines that would receive tokens from this offer is frozen. This could be the seller's trust line or the NFToken's issuer's trust line (if the NFToken has a transfer fee).
For a buy offer, the sender does have the funds specified in the Amount field available. If the Amount is XRP, this could be due to the reserve requirement; if the Amount is denominated in fungible tokens, this could be because they are frozen.
The NFToken has the lsfTransferable flag disabled and this transaction would not transfer the NFToken to or from the issuer.