What is an "E" vent?

[Ellypants]

I wasn't able to find anything about this online or in the forums so I'm hoping someone who reads this just happens to know!
I recently bought a used woodstove:

On the listed clearances, can someone tell me what an "E" vent is?
Appreciate any insight or information!

 

[begreen]

Double-walled stove pipe?

 

[Whitenuckler]

I wasn't able to find anything about this online or in the forums so I'm hoping someone who reads this just happens to know!
I recently bought a used woodstove:
View attachment 339816
On the listed clearances, can someone tell me what an "E" vent is?
Appreciate any insight or information!

I looked but did not score. I feel bad as the nameplate says Ontario.