In Sheets, with amount in column D, type the IFS that bands row 2 as "L" at 1000 or more, "M" at 300 or more, and "S" otherwise.

Answer

=IFS(D2>=1000, "L", D2>=300, "M", TRUE, "S")

Pairs of condition, value; the first TRUE condition wins, so the strictest test comes first. TRUE is the catch-all.