abs() vs abs[] for fby "while" all() vs all[]

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https://learninghub.kx.com/forums/topic/abs-vs-abs-for-fby-while-all-vs-all

May I ask how does abs() differs from abs[] in a select statement using fby clause? 

q)tab:([] sym:g1g3g1g1g2g3; price:-2.5 2 -0.5 -0.2 3 4)
q)tab
sym price
---------
g1  -2.5 
g3  2    
g1  -0.5 
g1  -0.2 
g2  3    
g3  4    
q)select from tab where abs(price)=({abs max x};price) fby sym
'type
  [0]  select from tab where abs(price)=({abs max x};price) fby sym
       ^
q)

and also similarly, in my qsql, if I use all with square brackets, it throws a rank error but all() works fine

select from xxx where all[col=a;col2=b] //rank error
How do I know when to use [] vs ()
Thank you!