9.99999999999999E+307 is the biggest number that Excel can handle (just under 1 with 308 zeros behind it).
9.99999999999999E+307 is used in formulas to find the largest value that is less than or equal to the search value. Because 9.99999999999999E+307 is an impossibly large value, the formula finds the previous numeric value.
Tips
You can probably use a smaller number than 9.99999999999999E+307, but make sure it is bigger than any number you might use in your file, for examlple, you can use 9.99E+307, but 9.9E+307 is not recommended.
9.99999999999999E+307 is often used with LOOKUP, MATCH, VLOOKUP or HLOOKUP function. For example:
Find the position of the largest value
The values in the lookup_array argument must be placed in ascending order.
=MATCH(9.99999999999999E+307,lookup_array,1)=
=MATCH(9.99999999999999E+307,lookup_array)Find the largest value
=VLOOKUP(9.99999999999999E+307,lookup_array,1,1)=HLOOKUP(9.99999999999999E+307,lookup_array,1,1)=LOOKUP(9.99999999999999E+307,lookup_array)