This is a mandatory field in MT700 type SWIFT msg.
This field indicates the date on which the LC is being issued by the Issuing bank.
According to SWIFT:
This field specifies the date on which the issuing bank (Sender) considers the documentary credit as being issued.
This is the issuance date, not the advising or acceptance date by the beneficiary.
Normally the SWIFT is transmitted on this date.
The format is: YYMMDD
Sometimes there’s an instruction in the LC to mention the LC issue date in the shipping documents.
Here’s an example: