difference between BT 709 (Not REC 709) and BT 2020

Hi everyone, just wondering if anyone can throw some light on the differences between BT709 and BT2020. I checked the internet but cannot see an answer. 

i shot a video in Vlog-L in 10 bit and it was captured in BT709 but when I edited on FCP under wide gamut HDR HLG and exported it, then it changed to BT 2020. Can someone tell me difference and if this will affect my HDR Quality/workflow.

Thanks in advance