Key Indexed Coupon in refers to an optional boolean that determines if the index associated with the input swaps should have its maturity adjusted to match the end date of the corresponding accrual period by setting accordingly the swaps' key
Look at the above link for more details.
If FALSE, the adjustment is carried out with the effect being that the reference period of the index is - almost - identical with the corresponding accrual period.
If TRUE, no such adjustment takes place and the index maturity may fall before or after the end of the corresponding accrual period.
If omitted, default is TRUE