You choose if you want 50/50 SBD/SP or 100 SP on any post you make. If you want to keep liqiud assets SBD you should choose 50/50 SBD/SP which will mean that in 7 days after you posted your content you will get half in SBD (half the dollar figure you see by your post), the other half will be in SP (calculated at the current valuation of STEEM converted into SP (Steem Power).

The other option 100 SP means your post would be payed out ONLY in SP which is great if you are in for the long haul. You can change for each post if you wish, It's good to use both, just try 50/50 on a post and try 100 percent SP on another, that way you can have both liquid assets and build SP too. Hope that is helpful to you.