What do the numbers in a name like L6 or H5 mean?
The letter is the chemical group of an ordinary chondrite (H, L or LL, reflecting its iron and metal content), and the number is the petrologic type, showing how strongly the rock was heated or altered by water on its parent asteroid.
Labels like H5 and L6 belong to chondrites, the stony meteorites that never melted. They pack two pieces of information into a couple of characters.
The letters describe chemistry. Ordinary chondrites are split into three groups. H stands for high iron, meaning the most total iron and the most free metal. L means low iron, and LL means low iron and low metal. Each group probably comes from a different parent asteroid or family of asteroids. Carbonaceous chondrites use their own two-letter codes, such as CM or CV.
The number, called the petrologic type, describes what happened to the rock after it formed. This scale was introduced in 1967 by the American scientists W. R. Van Schmus and J. A. Wood. Type 3 is the least changed: chondrules are sharp-edged and the minerals are varied in composition. From types 4 to 6, the rock was increasingly heated inside its parent asteroid, so minerals became more uniform, new crystals grew and chondrules blurred into their surroundings. A type 6 chondrite may have reached roughly 800 to 950 degrees Celsius without melting. Going the other way, types 2 and 1 were altered by liquid water rather than heat, with type 1 the most altered, its original minerals largely turned to clays. Type 3 is often further divided from 3.0 to 3.9 to capture very subtle changes.
Some examples: Peekskill, which famously hit a car in New York State in 1992, is an H6. Barwell, which fell in Leicestershire in 1965, is an L5. Chelyabinsk is an LL5, Murchison a CM2 and Orgueil a CI1. Two meteorites with the same label are similar in kind, but not necessarily from the same fall.