Look for the dropdown field at the bottom left.
If this query is referring to a hyper-specific online forum topic, or a video game mod archive labeled 21408.rar , could you provide it came from? This will allow for a more specific set of instructions! How to split rar files into parts 21408.rar
The extraction software will automatically read through Part 1, Part 2, etc., and stitch them back together into your complete file. Look for the dropdown field at the bottom left