I am used to working with Excel when the columns are referenced as letters
(A,B,C) and the rows referenced as numbers (1,2,3). However, some of the
files I have been working with lately are showing numbers for the rows AND
the columns. Therefore, the cell reference in a formula for cell "A1" is now
"R[-18]C[-3]".
What is this? How do I switch my files back to the "A1" format?

Thanks!