L10 is one of the three Ls - L10, L11, and L12.
L10 is also known as L 10, L-10, and the L10 Rundown.
What Is Scientology? says the Ls were developed in 1971.
L10 is extremely expensive, requiring a minimum of two intensives (12 1/2 hour blocks of auditing) costing thousands of dollars each. For example, Source 139 lists the Lifetime Member discounted price for L10 as $12,100 per intensive, for a minimum total cost of $24,200.
What Is Scientology? doesn't describe the individual L rundowns, but it does give a brief description of the 3 Ls as a whole. It notes that the Ls enable a person to exteriorize:
"... to leave the body and exist independent of the flesh. Exteriorized, the person can see without the body's eyes, hear without the body's ears and feel without the body's hands."
What Is Scientology?, 1998, p. 68
These three powerful OT rundowns, designated by the letter "L" - L10, L11 and L12, contain the only direct processes used to accomplish exteriorization. Audited by the most highly trained Scientology auditor, the Class XII, they result from research Mr. Hubbard conducted to discover why people who exteriorized on other processes did not stably maintain this state. Once this was established and resolved, he then examined why some people did not exteriorize at all. The result was the Ls. The Ls also bring other remarkable gains, allowing the being to fully realize his own innate power, putting him in a state in which he is unlikely ever again to suffer the effects of suppression, and, in truth, giving the individual a new life.