an attachment is any component/device not integrated into the design of the lift from the manufacturer.
now sideshifters used to be considered an attachment because they had to be installed on the carriage as a separate component and even to this day there are still 3rd party sideshifters that are considered attachments. If the sideshifter is integrated into the design of the lift bracket like alot are today it is not an attachment, it is considered part of the lift carriage/bracket assembly and part of the trucks mast assembly.

clamps, push-pull slipsheet, rotators, turn-a-fork devices, booms and components of that design would be considered attachments. And yes, "most attachments" do alter the capacity configuration of the lift so the lifts capacity data would have to be reconfigured and changed on the trucks dataplate to comply with manufacturer, ANSI and OSHA regulations.
Most baskets and platforms that are used in conjunction with the existing forks on the lift and are not installed permanently would not be considered an attachment and the dataplate would not have to be changed.

In most cases when the term 'attachment' is being used it generally refers to a device that is replacing the forks and is being used as a different method of manipulating or carrying the load/materials such as a bale clamp or rotating paper roll clamp or fork positioning sideshifter. (these are most common ones).