During a conventional path load, data fields in the data file are converted into columns in the database (direct path loads are conceptually similar, but the implementation is different). There are two conversion steps:
SQL*Loader uses the field specifications in the control file to interpret the format of the data file, parse the input data, and populate the bind arrays that correspond to a SQL INSERT statement using that data. A bind array is an area in memory where SQL*Loader stores data that is to be loaded. When the bind array is full, the data is transmitted to the database. The bind array size is controlled by the SQL*Loader BINDSIZE and READSIZE parameters.
The database accepts the data and executes the INSERT statement to store the data in the database.
Oracle Database uses the data type of the column to convert the data into its final, stored form. Keep in mind the distinction between a field in a data file and a column in the database. Remember also that the field data types defined in a SQL*Loader control file are not the same as the column data types.