Here you go....just copied from the electronic rulebook.
6:3 WINDOW NET
An SFI 27.1 ribbon-type or mesh-type window net is mandatory on
any full-bodied car running 7.49 (*4.49) or quicker. For full-bodied
cars running 7.50 (*4.50) to 9.99 (*6.39) or if vehicle runs 135 mph
or faster, a ribbon-type or SFI 27.1 mesh-type window net is
mandatory unless otherwise specified by Class Requirements. SFI
27.1 window net, when required, must be updated at two-year
intervals from the date of manufacture. Window net must be
securely mounted on the inside of the roll cage, with the permanent
attachment at the bottom. All attachment points must be designed
in an attempt to protect the driver and avoid contact with track
surface or guardwall. Eyelet clips, dogleash hardware, hose clamps,
etc. prohibited. Penetration of webbing, except as performed per
manufacturer’s instructions, prohibited. Any other modification to net
must be performed by manufacturer.