Dear Statalist

I read many papers estimate the CCEMG and AMG using the command of xtmg they add trend and robust , please can you explain it for me why it is important and when I need to use it ?

Example
Code:
 xtmg y X1 X2 , aug robust trend
Or
Code:
xtmg y X1 X2 , cce robust trend
When and why I can use the trend and robust in my analysis?

Thanks