When you validate mappings,

Data Integration

performs the following validations:

Verifies that the source and target fields in the task exist in the source or target. If the field does not exist, an error appears.
Verifies that all column data types are string and all field expressions contain string operations when the source and target are flat files.
Verifies that the correct parameters are used for each function and that the function is valid.

The expression validator does not perform case-sensitive checks on field names.

The expression validator verifies that the data type of a field in an expression matches the data type expected by the containing function. However, the expression validator does not check for incompatible data types between the following sets of objects:

Source and target fields of tasks.
Source field in a lookup condition and the lookup field.
Output of an expression or lookup and the target field.
The expression or lookup with these incompatible data types may validate successfully, but, at runtime, the task fails and an error appears.

If you map a string source field to a number target field, the validation succeeds.

Data Integration

tries to convert the string to a number using the atoi (ASCII to Integer) C function.

The expression validator does not validate lookups.