tuning versus resonance frequency

The resonant freq of the sub is not a limiting factor in tuning the box.  There are cases where the best result comes from tuning the box below Fs.  For example, a sub whose measurements and T/S parameters dictate that it should be used in a sealed box, but you choose to vent.  Oftentimes an unusually bad response curve occurs when the box is tuned to "normal" car audio ranges.  To get best response, a lower tuning freq is chosen.  Read this recent thread for a good example of this phenomena.

You will simply get little port output when the box is tuned like this.  But venting the box isn't always all about increasing SPL by adding the port response.  In the case as linked above, it's to get some deeper sub base from the sub that the sealed box wouldn't allow.  But the trade-offs can be a put-off.

The box tuning, on the other hand, once chosen and implemented becomes the limiting factor.  At below Fb (box tuning freq), the box has no control over the movement of the sub.  So when these lower sub tones arrive, the sub movement will be out of control.  This out of control flapping can easily damage the sub.  You could have a sub with a Fs (resonant freq) of 24 Hz in a vented box tuned to 40 Hz, and the sub can be damaged with tones well above its Fs.  The damage isn't done because you've allowed tones to arrive below Fs, but because you didn't limit the tones below box Fb.  So a strong signal at 34 Hz could wack your sub out.

Most vented sub applications require the use of a subsonic filter to keep out these too-low freqs.

Recommended:  learn all you can and enjoy your vented box-building experience.  I would say with all modesty that the sticky above "Guide: Using WinISD" is a good way to start.