Below is from "The little SAS book".

It will scan your data file (the first 20 rows by default, the EXCEL identifier only looks at the first 8 data rows by default, XLS identifier looks at all rows in the file to determine the column type) and automatically determine the variable types (character or numeric). If the first 20 rows data are missing, we can use OPTION “GUESSINGROWS=” to define how many rows we are using to determine the variable type.