The summarization is based on SAS Online Tutor.

Automatic character-to-numeric conversion occurs, when a character value is -- assigned to a previously defined numeric variable
-- used in an arithmetic operation
-- compared to a numeric value, using the comparison operator
-- specified in a function that requies numeric arguments (e.g. question 4 in SAS Base 70).
The automatic conversion may produce a numeric missing value from any character value that does not conform to standard numeric notation that contains only digits with an optional decimal point or leading sign (e.g. question 10 in SAS Base 70).
In other situations, a character value will not be converted to a numeric value automatically (e.g. question 19 and 68 in SAS Base 70).

Automatic numeric-to-character conversion occurs, when a numeric value is
-- assigned to a previously defined character variable
-- used with an operator that requires a character value
-- specified in a function that requires character arguments.