[SOLVED] What does THETA (greek letter) mean in electronics?

[harvie]

Surge protector T2+T3 Hakel HSA-275/4+0 M has following function diagram in it's datasheet:

At lower side there seems to be schematic symbol for something resembling varistor. But what is the thing on top?
Switch with something that looks like greek cursive letter "theta".

Any idea what that might be?
I have feeling it's probably just plain old fuse. But the image seems to be confusing.
Some websites say that theta is sometimes used for "angle" in physics.

Thanks

 

[wwfeldman]

page 1 of the data sheet lists theta as operating temperature, -40 to 70 degrees C

the web page for said device https://www.hakel.com/en/product-hsa-275-detail-153948
has quick contact information with a phone number and an email address.