Key Floors 1 in refers to an optional of predefined constant numbers f that serve as the lower bound the applicable fixing in Index 1 may assume in order to produce the final rate used in calculating the floating cash flow amount.
If omitted, no lower bound is assumed.
If n values are provided, then the first n cash flows are paired with the provided n values in successive order.
If any cash flows are left over, they will use the last value.
An undefined value is interpreted as an absence of an lower bound for the respective rate.
One or more numerical elements of an array may be marked as having an "undefined" value by letting them containing the text N/A
Undefined dates can be marked throught the text [No Date]
Alternatively in Excel, when these elements precede a regular (i.e. "defined") element, they can be more conveniently represented by blank cells.
Also in Excel, the special text [end] can be used to mark the end of an array.
Look at for the exact mathematical formula.