splituapp is similar to splitupdate.pl and split_updata.pl for unpacking/splitting/extracting img files from an UPDATE.APP file. The main difference being this one is written in Python2/3 instead of Perl. Additionally, splituapp allows you to extract specified img files where the others will only extract all img files at once. This saves a lot of time if you know which img files you want.

Assuming splituapp and UPDATE.APP are in the same directory and you have a command prompt open in that directory, here are some examples.

To extract all img files:

splituapp -f UPDATE.APP

To extract only system.img and boot.img:

splituapp -f UPDATE.APP -l system boot

You can run ./splituapp -h (Linux) or python splituapp -h (Windows) to get usage instructions for the OS you are running on.
Like splitupdate.pl and split_updata.pl, splituapp will do crc checksum verification on the img files after extraction if the crc binary is in the same directory the script is run from. Unfortunately I do not have the source for the binary so it is only available for Linux at this time. As a result, the crc verification will be disabled in Windows for now.

splituapp does not extract the images it splits from UPDATE.APP. Once the img files are split from UPDATE.APP the job is complete. If you are looking for a tool to handle system.img extraction, you can check out SuperR's Kitchen.