Why some global parameter is in { } while the others are not?

Dear all,

I'm helping my prof modify past code to adjust to the new dataset. It is my first time using globle parameter to navigate through files. I have a separate dofile for all global parameters, and inside there is this global parameter:

Code:

global weight_var                                 weight_worker

And in my statistic calculating do file, when use this parameter, it write

Code:

keep $wage_definition_ts ${weight_var} $gender_groups

, while you can see all other global parameters are not in { }.
I want to ask what is the difference if I put it in {} and not?

Thanks!