Imperial says it protects walls and ceilings where clearances are short, on 5 to 8 inch pipe.
Using it
Each elbow costs draft, so keep the run simple. Fix joints with screws as your manual directs, and keep clearances from walls and ceilings.
Questions this guide leaves you with
Is it an elbow?
No, it is a shield that fits over one.
Does it make any clearance legal?
No. Follow your manual and local code.
The price on Amazon.com
This model is listed on Amazon.com, at ≈ $34 at the last reading. Listed is not the same as in stock: availability changes daily, and the buy box shows the live price.
At $34, it sits +3 % against the $33 median of the 10 models in this category published here.
Sources
Specifications: the manufacturer sheet for wood heating equipment and the matching Amazon product page. Prices, ratings and availability: read on Amazon.com from a New York address. Every figure on this page names its source.
The price beside each product is fetched as this page opens, and it is the only figure here that is current. Any amount written into the prose was read once, on the day that sentence was written, and nothing has refreshed it since.