The "SSL Key" is the private key (private.key.pem).
An "SSL Request" is probably a key signing request/certificate signing request (CSR), which is the file you send to the Certification Authority in order to obtain a certificate. There's no reason why you should have to put a CSR into ISPConfig when you've got a signed certificate already.
The "SSL Certificate" is obviously your certificate (domain.cert.pem).
An "SSL Bundle" is a file containing the intermediary certificate(s) between your certificate and the CA Root Certificate (intermediate.cert.pem). It is common practice for web servers to serve these certificates to clients accessing the domain, because the clients will then be able to validate the certificate chain all the way down to root, without having to first somehow obtain the intermediate certificates.
That leaves only "public.key.pem", which I assume is just the public key corresponding to the private key in private.key.pem. Since the public key is embedded in the certificate, the information in this file is redundant.
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