In[]:=
Subsequences[{a,b,c,d,e,f}]
Out[]=
{{},{a},{b},{c},{d},{e},{f},{a,b},{b,c},{c,d},{d,e},{e,f},{a,b,c},{b,c,d},{c,d,e},{d,e,f},{a,b,c,d},{b,c,d,e},{c,d,e,f},{a,b,c,d,e},{b,c,d,e,f},{a,b,c,d,e,f}}
In[]:=
Subsets[{a,b,c,d,e,f}]
Out[]=
{{},{a},{b},{c},{d},{e},{f},{a,b},{a,c},{a,d},{a,e},{a,f},{b,c},{b,d},{b,e},{b,f},{c,d},{c,e},{c,f},{d,e},{d,f},{e,f},{a,b,c},{a,b,d},{a,b,e},{a,b,f},{a,c,d},{a,c,e},{a,c,f},{a,d,e},{a,d,f},{a,e,f},{b,c,d},{b,c,e},{b,c,f},{b,d,e},{b,d,f},{b,e,f},{c,d,e},{c,d,f},{c,e,f},{d,e,f},{a,b,c,d},{a,b,c,e},{a,b,c,f},{a,b,d,e},{a,b,d,f},{a,b,e,f},{a,c,d,e},{a,c,d,f},{a,c,e,f},{a,d,e,f},{b,c,d,e},{b,c,d,f},{b,c,e,f},{b,d,e,f},{c,d,e,f},{a,b,c,d,e},{a,b,c,d,f},{a,b,c,e,f},{a,b,d,e,f},{a,c,d,e,f},{b,c,d,e,f},{a,b,c,d,e,f}}