userid = id of the users, see sys_user table.
groupid = The groupid if the user, see group table.
doctype_id = doctype ID, see doctpye table
tree_id = primary ID of the *_nodes table
type = type of the node. There are two types, one for nodes and one for folders.
doc_id = the doc_id of the object were this node is for, see isp_isp_web, isp_isp_domain etc.
Are the Isp_fakt_nodes and isp_fakt_artikel the only tables used for the invoiceing ?
everything with isp_fakt_* is for the invoicing.
Do you have any documentation how your billing system functions ? Flow charts ? anythin ?