Unusual HVAC Vent

Did an inspection today and noticed a strange vent pipe from a natural gas furnace in the attic. It looked very similar to a flexible dryer vent pipe. I have never seen this in my area but wanted to get a second opinion before noting a defect. They were about five feet long then went to a metal coupler and back to the flexible line. Attached are a couple of photos, can anyone help?

I Learned something new.

Thanks Bob that was what I was looking at. It did go back to rigid metal through he roof sheathing, however it was installed horizontal and sagging in several locations.

Yes the unit was a mid-efficency.

Thanks again for all the help.

Still needs correction then and remember the 1" clearance rule.

Just an additional thought,seems the flexible B-vent is only available up to 5 foot lengths,looked longer in the pic.

What’ the benefit of using this B-flex ? easy to install?