are offered by a number of Train Companies on selected flows as a bulk-buying product, providing a discount over purchasing individual tickets. These products offer a cheaper alternative for some passengers who do not travel frequently enough to be able to make good use of a season ticket.
A carnet typically offers a discount of at least 10%.
There is a comprehensive guide to carnets here
5.13.1 Time Restrictions
Some Train Companies offer both an Anytime and Off-Peak version whereas others only offer carnets on Anytime tickets. Each ticket in a book of carnet has the same time restrictions as if purchased individually. Some carnet products have different names.
Please see the link for each individual product for more details:
5.13.2 Ticket Issuing
Individual tickets in a book of carnets should be printed on separate coupons on standard railway stock, or should come in the form of tear-off coupons in a book. Each ticket must be validated prior to travel by inserting the date of use in the date boxes provided. This validated ticket is then used in the same manner as an individual ticket.
It is advisable to use a non-scratchy pen, preferably a fine-point permanent marker, to fill in the date boxes on these tickets.