Meaning of some radar contact symbology?

[Lymark]

Colors of the contacts are self-explanatory, but I'm not sure what the different shapes/sizes mean. So far I have seen:

 

Unknown(Yellow): 1) Solid square block, 2) Hollow and larger diamond

Friendly (Green) : 1) Hollow triangle 2) Hollow circle

 

Sometimes, I've even seen different shapes of yellow and green on the same target. Is that some sort of bug?

 

Thanks!