Ettercap target question

[HDD2]

Alright folks, getting a little confused while trying to use ettercap. I do honestly prefer the CLI over GUI, and everything seems fine and dandy until i see the '// //' I've tried MiTM with only 1 '//' and it seems to work just fine. I read up in the man pages that '//' designates 'MAC/IP(s)/PORT', but what are the targets actually used for? Is the first '//' saying "take info from THESE machines" and the second '//' saying "send recieved info to THOSE machines"? Or, are the two '// //' just giving you the options to specify 2 machines to do a MiTM attack on? This is very hard to explain, i apologize for the confusion. If someone could break down exactly why there are 2 targets, what ettercap is actually doing with these 2 targets, and specifically how to use these targets...you'd definitely make my life a little easier. Thanks everyone.

EDIT: Like what would the difference between "/192.168.x.x/ //" ...and "// /192.168.x.x/" be?