Decking Board Question

[Brian in sunny FL]

I have a local guy selling 5/4 x 6" x 16' western red cedar decking boards for $16 each which seems like a pretty good deal.  I have some that need replacing and was going to use synthetic but may just use these based on the price and planned on building some large planters so can plane them down and use same material.  Two questions.

They are rounding on one corner, is that the top side or bottom side to hide fasteners.

Can you use hidden fasteners on any board or do they need to be made for it?  Which ones are popular?