Hello,
The param syntax for both select and multiselect is technically identical. The "::" means simply that this param should assume default value. As you probably already know param syntax is defendant on field type. crm_contact and crm_company types have a little different order for those parameters.
But in general, the meaning of each segment for select/multiselect elements is:
recordset element is pointing to :: field name the element is pointing to ;
Class name :: method that returns criteria for records ;
Class name :: method that return more advanced properties for the field
To set those parameters, just assign a value under 'param' key in Installation file, in relevant field row.
Alternatively you can also define QFfield_callback and in that method define all the field constructing rules by yourself (this however, is a bit more advanced approach).
Hope this helps,
Arek