These speakers are designed for a 100V loudspeaker distribution system.
There are 3 common loudspeaker distribution voltages: 25V, 70V, 100V. North America uses mostly 70V systems, with a very few systems using 25V distribution. I think that 100V distribution is used in places like Japan and Europe but I've never worked with any of those systems. Regardless, the principles are the same no matter what the target voltage is.
The concept behind constant-voltage loudspeaker distribution systems is simple: it's too much work to figure out how to match the load impedance to the amplifier. It's much easier to design the amplifier to output 70 Vac RMS when it is operating at full power. It is then a simple matter to use a transformer to drop that voltage such that the desired amount of power is fed to an individual speaker.
Most loudspeakers designed for use with constant-voltage distribution systems have multiple taps so that the amount of power being fed to that particular speaker is made appropriate for the space it is being used in. Most common taps on the small speakers that I've worked with are octave steps starting at 1/4 Watt and going up to the power rating of the speaker. That is: 1/4W, 1/2W, 1W, 2W, 4W, 8W, etc.
Conversely, larger speakers start at the full rated power of the speaker and work downwards: the Community W2-218WT speakers installed recently at our local football stadium have taps at 100W, 50W, 25W, 12.5W.
In general, speakers that are designed for use at 100V can also be used at lower voltages but you have to recalculate the power that is available at each tap. However, speakers should not be used on a distribution system that has a higher voltage than the speaker rating - the transformer may saturate and bad things can happen.