First - are you talking about more than 1 domain? I mean first simplify first the things that you want to do - make it clearer.
How to use domain? Domain is like your brand, so your domain name must sounds professional, catchy, and it would be best paired with .com, .co, .net - for example: cashoutdecently.com (well, that's mine).
Why people use .net, .org. .co for the same domain name? Say cashoutdecently.com, as well as cashoutdecently.net, and cashoutdecently.co -
That is one technique to catch out traffic from spilling outside your basket. Sometimes people type .co instead of .com. But take note, all the other here are redirected to one - say your site has the real domain of yoursite.com!
How about subdomains? If you are dealing a lot, or if you have a multiple niche, then you might use subdomains, it's perfect for a sub-categories of your site or brand. Say, if you services, then you can put this way - services.yoursite.com; if you have forums, then you can put this way forums.yoursite.com, if you have blog, then blog.yoursite.com, if you have free coupons, then coupons.yoursite.com, if your have support, support.yoursite.com and much more....
So now - decide what to do first - plan it further