Engine Blow By

Ive been looking at some used Cat machines descriptions.

and in their condition notes they mention the 'blow by', some of the descriptions will say



Blow By - Yes - Normal

or

Blow By - No



If a machine has any kind of 'blow by' is it asking for trouble?



If some have NO 'blow by', which im assuming is normal... than how can you have 'blow by' and have it normal?



im lost....