lookup2 and lookdown2 are upgraded version of lookup and lookdown , which you can use variables with them inside the brackets but with the old version you can't

1) Lookdown2 is used to search for specific value between the brackets and return its index for example here is a sample to conver hexadecimal into decimal

lookdown2 hexadecimal,[0,1,2,3,4,5,6,7,8,9,$A,$B,$C,$D,$E,$F],decimal

so if hexadecimal = $A ==> decimal =10 ( the index in brackets begin from 0 for 0 to 15 for $F )

2) Lookup2 is used to get the data of specific index you determine

for example

lookup2 index,["hello",10,$A,"H"],char

------ so
if index=0 then char="h"
if index= 5 then char=10

and so on

I hope these information is clear