IN-S42ATR is the red member of Inolux's IN-S42AT series: a low-profile 0402 top-view indicator LED in a clear, PCB-type moulded package.^p.1 Electrically it is an unremarkable AlInGaP red emitter, which is the point — it is the part you reach for when a board needs a power or status light and nothing about the light itself should be interesting.

Budget 2.1 V across it at the 20 mA test point and it returns 90 mcd.^p.3 Drive it through a current-limiting resistor rather than straight off a rail — the datasheet says so explicitly^p.12 — and keep continuous current at or below 25 mA.^p.2 At 3.3 V that is a (3.3 − 2.1) / 0.02 = 60 Ω resistor for the full 20 mA, and most indicators are happier an order of magnitude dimmer than that.

Two limits are worth remembering because they are tighter than instinct suggests. The 130° viewing angle^p.3 makes this a wide-angle panel indicator, not a directional light source. And reverse voltage is rated to only 5 V^p.2 — well under a 12 V rail, so it must never be left reverse-biased across one.