Enabling logminer

Hello all,

Could you please help me 

what is the purpose of

"ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;"

In our rac environment, we are often getting blocking session and customer is asking us to enable the logminer by using above command.if enabling do we need a lincese for this

I donot understand for what purpose they are going to use the above statemnet.

I try to find some info about the above statement,but i didnot get any info even in metalink .

please suggest me .

Thanks in advance

Nitya