ANSI refers to them as either "work positioning lanyards" (Z 133.1: 3.16 and 8.7.2) or "personnel lanyards" (B.4.4).
r/c is basically correct. The Microcender falls short of the 5,000 pound minimum breaking strength required by ANSI Z 133.1. I have looked into this before and the MBS for the Microcender is reported as somewhere between 4,000 and 4,500 pounds. Petzl's Micrograb (originally sold by Rock Exotica as the Microjuster) is made specifically as a lanyard adjuster for the tree industry and is essentially a beefed up version of the Microcender. The Micrograb (6 oz.) is one ounce heavier than the Microcender (5 oz.), but exceeds the 5,000 pound MBS and has a shoulder bolt and lock nut instead of the quick release pin that the Microcender has. See Blair's 'Arborist Equipment', pp. 112-113.
That may all be moot, however, because when shock loaded the units will slip at around 4 kN (900 pounds). Depending on the type and diameter of the rope, these devices may even shred or shear the rope at shock loads that are lower than the MBS.
Mahk