Family Tree DNA prefixes kit numbers with one or more letters depending on their source.

None – Directly ordered from Family Tree DNA.
A – African DNA
AM – Unknown
B – Transferred Y-DNA or autosomal DNA results from other testing companies including 23andMe, AncestryDNA, and Sorenson.
E – iGENEA
H – DNA Heritage
K – Shejire DNA (Kazakhstan)
M – DNA Ancestry & Family Origin
N – National Geographic Genographic Project
T – Libya DNA Center
U – DNA Worldwide
V – Jewish Voice
Y – Akira Yamakazi
Z – Genera

Sources & Resources: ISOGG Wiki – FTDNA kit numbers