Installation of cURL and curlSSL - curlSSL missing?

Might be a silly question (not fully clued up on how the php extensions work) but on my server I have cURL installed for all version of my PHP however a client of mine wants me to install the XERO plugin to link their website to their invoicing and its minimum requirement are that cURL and curlSSL are installed. Is curlSSL an additional extension? Or is linked to the website having an SSL attached to it? Any pointers appreciated. Thanks, Will