You can either override to the required member before starting your RPG program or you can spcify the member on your F specs in the RPG program. Check out the extmbr() keyword for F-specs.
Yeah, just know that if both an OVRDBF and the ExtMbr() are used, the OVRDBF takes precedence.
Note that the parameter to EXTMBR() can be a field name. If the file is opened automatically (no USROPN), you need to make sure the field has a value during initialization. For example, it could be an incoming parameter, or the field could have an INZ.
If you have EXTMBR() with a field, with USROPN, you choose when to OPEN the file. In this case you need to make sure you set the field's value to the member name before you do the OPEN. You could later CLOSE the file, change the field, and reopen with a different member name.