In FTDNA, it is the sequencing of a part of the mitochondria adn in the kit.http://en.wikipedia.org/wiki/Mitochondrial_DNAmitochondriahttp://en.wikipedia.org/wiki/Genealogical_DNA_test
Mitochondrial DNA (MtDNA) is transmitted from mother to children, therefore a matrilinear transmission,
mtDNA, by current conventions, is divided into three regions. They are the coding region (00577-16023) and two Hyper Variable Regions (HVR1 [16024-16569], and HVR2 [00001-00576]).
FTDNA company has 3 different tests according to the present financial possibilities of the customer :
mtdna : only HVR1 sequenced
mtdnaplus : HVR1 + HVR2 sequencing.
mtfullsequencing HVR1 + HVR2 + coding region.
to allow the best knowledge of your matrilear lineage, mtfullsequencing is a lot preferable , but if you are not rich, you could test region by region.