99 reflects the potential length of a string between the 1st comma and the second comma

Substring syntax

Substring (expression,startpoint,lenght)

99 represents thee Length of the expression

build the query up have a play with substring

here another example with multiple results

create table col (col varchar(255))

insert into col select 'A,B,C' union all

select 'AA,BBBB,CCC'union all

select 'AAA,BBBBBBBBBBBBBBBBBBBBBBBBBBBBB,CCC'union all

select 'AAAAAAA,BBBBBBBB,CCC'

Select substring( SUBSTRING(col,charindex(',',col)+1,99),0,charindex(',',SUBSTRING(col,charindex(',',col)+1,99)))

from #t1