PTT is an acronym of Pots Tubs and Trays and is used in relation to, as examples, yogurt pots, margarine tubs and trays for meat and/or cheese. It is a plastic 'grouping' generally captured as PS (6) in WikiWaste but can also include PP (5) and PET (1).

Image of Pots Tubs and Trays from Wales Recycles website, all rights reserved
Image of Pots Tubs and Trays from Wales Recycles website, all rights reserved