MailChimp merge_vars what else is accepted apart from FNAME & LNAME?

I have implemented a very basic sign up using email address+name, although I would now like to add extra data such as Phone Number, Website etc.

My question is where in the API doc can I find what else is supported apart from FNAME AND LNAME?

At the moment I have:

$merge_vars = array(
     'FNAME' => $fname,
     'LNAME' => $lname
);

I am using the method listSubscribe and this is the information that is provided above:

@param array $merge_vars optional merges for the email (FNAME, LNAME, etc.) (see examples below for handling "blank" arrays). Note that a merge field can only hold up to 255 bytes. Also, there are a few "special" keys:
                        string EMAIL set this to change the email address. This is only respected on calls using update_existing or when passed to listUpdateMember()
                        array GROUPINGS Set Interest Groups by Grouping. Each element in this array should be an array containing the "groups" parameter which contains a comma delimited list of Interest Groups to add. Commas in Interest Group names should be escaped with a backslash. ie, "," => "\," and either an "id" or "name" parameter to specify the Grouping - get from listInterestGroupings()
                        string OPTINIP Set the Opt-in IP fields. <em>Abusing this may cause your account to be suspended.</em> We do validate this and it must not be a private IP address.
                        array MC_LOCATION Set the members geographic location. By default if this merge field exists, we'll update using the optin_ip if it exists. If the array contains LATITUDE and LONGITUDE keys, they will be used. NOTE - this will slow down each subscribe call a bit, especially for lat/lng pairs in sparsely populated areas. Currently our automated background processes can and will overwrite this based on