I think it is very important that your letters of reference specifically refer to roles and responsibilities associated with those in the noc class. there is no problem if all criteria are not fully mentioned in the letters but the bones of them must be noc related. for example you could have a carpentry background but as long as you can show that you managed a team of carpenters on a site for a time and was responsible for their work, the programme,safety,quality and budget (all to a certain extent) then noc 0711 can be applied for. If however you were employed as an engineer and only get a letter from your employer to that extent, this might not suffice. Its very important that the letter makes reference to roles and responsibilities carried out that relate to the noc you are applying for. I hope this helps