Availability of Letters of Credit
A credit must state whether it is available by sight payment, deferred payment, acceptance or negotiation. (UCP 600 - Article 6- b)

Sight Payment : Sight payment refers to the payment which is made as soon as the complying presentation is seen by the issuing bank or the bank nominated in the letter of credit. The nominated bank fulfills its payment obligation with recourse basis.  Nominated bank can demand the amount paid to the beneficiary back in case of documents are found noncomplying by the issuing bank. Nominated bank's payment obligation is not strict as the issuing or the confirming bank's payment obligation. UCP 600 states that a nominated bank is not obligated to accept the nomination directed to itself unless nominated bank inform its acceptance of nomination expressly to the beneficiary. Even in this situation UCP 600 assumes non payment by the nominated bank and describe the roles of the issuing and confirming banks.
Deferred Payment : Deferred payment refers to the payment which is made after a period of time that is specified also in the letter of credit. The payment period is usually determined as specific number of days after the date of presentation or the date of the transport document. Bill of exchange or draft is not required under deferred payment.
Acceptance : Acceptance refers to acceptance of a bill of exchange which is drawn on the bank mentioned in the letter of credit to be presented with the other required documents and payment at the maturity. 
Negotiation : Let us check the definition of the negotiation from the UCP 600 in order to understand the term more clearly.
Negotiation means the purchase by the nominated bank of drafts (drawn on a bank other than the nominated bank) and/or documents under a complying presentation, by advancing or agreeing to advance funds to the beneficiary on or before the banking day on which reimbursement is due to the nominated bank. (UCP 600 - Article 2)
As can be seen from the above definition any letter of credit which is available by negotiation could be issued in a way so that a draft may or may not be required. Also the payment of the letter of credit may or may not be at sight. If the nominated bank advance or agreeing to advance funds to the beneficiary before it receives reimbursement from the issuing bank the negotiation condition is fulfilled. This can happen in two ways. First possibility is, in a letter of credit which is available by sight payment when the nominated bank reimburse the beneficiary before it receives funds from the issuing bank. Second possibility occurs when a letter of credit is available by negotiation requesting presentation of a time draft, while the nominated bank reimburses the beneficiary before the maturity date of the bill of exchange. (How does a negotiable letter of credit work?)

When issuing a documentary credit via swift message banks show the availability of a documentary credit in Field 41a. 
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What does availability mean in a letter of credit transaction?
In documentary credit terminology, availability refers to the availability of the documents in exchange for the payment of the amount stated in the letter of credit. UCP 600 defines four availability options ;
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MT 700 Issue of a Documentary Credit 
      Field 41a: Available With ... By ...
Payment means payment at sight. Mixed payment is not directly covered under UCP 600.