For export operations, you can specify dump files at the time the job is defined, and also at a later time during the operation. For example, if you discover that space is running low during an export operation, then you can add additional dump files by using the Data Pump Export ADD_FILE command in interactive mode.
For import operations, all dump files must be specified at the time the job is defined.
Log files and SQL files overwrite previously existing files. For dump files, you can use the Export REUSE_DUMPFILES parameter to specify whether to overwrite a preexisting dump file.