Bit depth is the number of bits of information in each sample. A higher bit depth equals higher resolution for each sample. The default Logic Pro setting is 24-bit recording. If you turn off 24-bit recording, Logic Pro will record in 16 bit. Similar to your sample rate, the only downsides of using a higher bit depth are that it takes up more drive space and uses more processing power.
To turn on or off 24-bit recording:
Choose Logic Pro→Preferences→Audio.
The Preferences window opens to the Audio pane.
Click the Devices tab.
Select the Core Audio check box.
In the middle of the screen, select or deselect the 24-Bit Recording option.
When 24-bit recording is not selected, Logic Pro records in 16 bit.
Bit depth is different than bit rate, which is the number of bits of information processed each second, as opposed to each sample. When exporting your project to lossy audio formats such as MP3, higher bit rates equal higher quality sound.