Rain cap/spark arrestor fits on top section of pipe.
When ordering a Yukon or Wilderness stove with 5" straight pipe, the 6" rain cap in a stove package will not fit on the 5" straight. You will need to order a 5" rain cap id you decide you want a rain cap.
Rain cap prevents rain and snow going down your stove pipe.
Spark arrestor prevents most ash and sparks landing on tent roof.
Size of rain cap/spark arrestor is determined by diameter at the top of last section of pipe.