They're campaign badges.
In 2nd edition Space Marines had a campaign badge for the current campaign they had embarked on, usually (and at least for Ultramarines) on the back of the leg greave. Transfer sheets generally gave you several options, but also the studio armies tended to stick to the same one per chapter (so they could mix and match the models more than anything) - the skull in the red triangle was used on Dark Angels, the black skull in the white circle was used on the Ultramarines, the simpler lightning was used on Blood Angels and the more complex on Space Wolves. I think this sheet may have just had the wing for extra variety.
However, when they did the new Marine plastics for 3rd (I think? Christ) ed, that section of the leg had too much molded detail to apply them cleanly, so their use fell out of favour with most painters.
I think the yellow lightning bolt is an honour marking for Blood Angel officers.
Over the years some were reused for less famous chapter markings, as there were loads of easily available transfers out there.