Operating System Concepts discusses spooling in the following.
"A spool is a buffer". Is the spool buffer in memory?
"Each application’s output is spooled to a separate disk ﬁle." Are spool files residing on the spool "buffer" or on a disk?
Does the spool buffer work like a memory buffer for the spool file on disk?
A spool is a buffer that holds output for a device, such as a printer, that cannot accept interleaved data streams. Although a printer can serve only one job at a time, several applications may wish to print their output concurrently, without having their output mixed together. The operating system solves this problem by intercepting all output to the printer. Each application’s output is spooled to a separate disk ﬁle. When an application ﬁnishes printing, the spooling system queues the corresponding spool ﬁle for output to the printer. The spooling system copies the queued spool ﬁles to the printer one at a time.