Liners will typically range in size from a 36 cell tray up to a 288 cell tray, with 50 and 72 being most commonly used by wholesale nurseries. The term "Liner", is typically used when referencing perennial, ornamental, and woody type material.
Annuals in this form are usually called plugs though. Commercial growers typically get these in 128 cell trays all the way up to 512 cell trays (very small), while the the bigger one like 36 and 18 cell trays are offered mail order and direct sales.
In general it is a term used to describe a young plant that will be re-potted into a larger container for finishing.
A generic term for any plants used to grow on to a larger size, usually refers to rooted cuttings. Differs from a plug, which is a specific type of liner grown in a tray division, often from seed. It is bareroot, a term that means NO dirt around the rootball; wholesale plants may arrive bareroot for further processing.