If the original archive was encrypted, it uses AES-256 bit encryption. Why Archives are Split Splitting is a standard procedure for several scenarios:
Do you have the (001 through 005), or are you trying to recover data from a missing segment ? Frequently Asked Questions (FAQ) - 7-Zip Flow.7z.005
Ensure you have every segment in the same folder (e.g., Flow.7z.001 , Flow.7z.002 ... through at least 005 ). If the original archive was encrypted, it uses