The L-spec was a line counter specification. It was used with a program described printer file to indicate the form length and number of lines to print on a page.
It is an obsolete specification and is no longer part of the modern RPG language (RPG IV). Although it is still supported in RPG III, I never used it and the very few programs in which I vaguely recall having seen it were very old.
For further information, here is the RPG III / RPG 400 manual – see chapter 7.