I have a comparable "hockey puck" SSR handy. It also has the 3V to 32V input, although it's a different model (ESR5102401000Z). I made some quick measurements.
I was curious how these solid state relays (SSR) manage to accept a control voltage with a fairly wide range: from 3V to 32V. Of course, the datasheets for these "hockey puck" SSR don't provide the details about the input side. I can think of 3 schemes. Different models of SSRs may use different schemes.
Current limiting resistor
The LED and resistor are such that the current is low enough at 32V not to destroy the LED, but still high enough at +3V to activate the relay. In this arrangement, the input current will increase linearly with input voltage.
Series current limiter (active)
The input current shouldn't vary much with input voltage. The constant current source may be more or less stiff. The plot in Spehro's answer suggests a series constant current regulator in his SSR.
Shunt current limiter (active)
The input current will increase linearly with input voltage. Excess current shunted around the LED, but still drawn from the input. Here's an example of what a shunt current limiter may look like (source).