Bill lines. Allowed lineType: title, element, comment. Two title levels are allowed: a title's content can hold elements, comments and sub-titles (themselves title lines), but a sub-title's content can only hold element or comment lines, never another title (titles are nested at most two levels deep). Allowed fields per line: label, quantity, unit, netUnitPrice, taxRateId, elementId, lineType (plus content on title and sub-title lines). element lines require quantity, unit, netUnitPrice and taxRateId. When an elementId (a catalog element or structure/ouvrage) is provided, only quantity is required: unit, netUnitPrice and taxRateId are inherited from the element when omitted (any value you pass takes precedence). If the elementId is a structure (ouvrage), its composition is loaded and saved into the bill.