What would this kind of device be called

The ONLY source of reliable data in in the data sheet. I could not see this parameter in the data sheet for that device. It looks like some sort of data extraction algorithm that has gone wrong.
Contact the distributors and ask them about it.

But as I said it might be the maximum voltage that ensures pull in as seen by this section of the data sheet:-

coil.png

Note is says it is 70% of the nominal coil voltage which at 5V is 3.5V.

Thanks for the pic, helps a lot for understanding. I'm taking the 3.5v MAX turn-on as a manufacturing spec indicating a threshold of tolerance. Meaning, "the bottom end of voltage that the relay will be responsive to is at MOST 3.5v" because a lower threshold is preferable to ease of triggering whereas a relay that has a tiny turn-on voltage range of just 4.5 to 5 is more demanding. In plain English it still sort of reads like a minimum, but basically "the minimum voltage to turn on the relay will be at most 3.5v". Sounds wonky reading it out but it makes sense in my head, which is all that matters to me. :)

Not interested in latching relay modules anymore? If so, I gave some links in reply#11.

I was looking at modules, looking at the relays they had onboard, and was trying to understand the specs on them. Still looking for modules, but again for understanding sake I'm trying to learn about the components that are on those modules.

I understand why those components are necessary for standard non-latching relays, but for latching/impulse/bistable relays, it seems the power requirements are much smaller, which is why by the numbers, it sounds to me like they could be run by an Arduino digital pin without all those module components.

So I'm looking at both. Modules if I'm to understand I need those other components, unmounted relay units if I'm to understand I don't.

I'm not aware of any relays that can be directly connected to an Arduino pin. Most all need extra components to drive the coil and clamp the fly back spike. Latching relay coils are designed for a quick impulse, but they still (temporarily) require more current than an Arduino pin can provide.

So, what you're saying is, I should scrap the whole idea of a relay and use a stepper motor that CAN be powered by the Arduino directly with a lead strapped onto the spinny part and make my own latching switch? :grin:

Just saying that if you buy a relay, you'll need extra components to control it. If you buy a relay module, you can control it directly with Arduino pins.

In plain English it still sort of reads like a minimum, but basically "the minimum voltage to turn on the relay will be at most 3.5v".

Yes that is it. So the relay might turn on at a lower voltage but by the time you reach 3.5V it is guaranteed to turn on.

a 5V relay is one that you'd use with a control circuit having a 5V power supply. Since such a circuit normally includes a switching transistor or even a darlington driver with an INHERENT voltage drop of about 0.2 to over 1V, it's pretty important that such relays be able to be used with a coil voltage that is somewhat less than a full 5V. I've seen relays spec'ed for coil currents that are not much more than a bare Arduino pin can supply; certainly two paralleled pins... (the NAIS TX2SA-L2-5V I linked to earlier is spec'ed at 40mA coil current.) You'd still need some additional circuitry to handle the reverse voltage spike...