KX3 sent PSK31 letters lower or upper case?

[ve3ibw]

Is the KX3's PSK D support for sent PSK31 letters upper or lower case?  I ask because the VFO B display on the KX3 shows only upper case letters being sent and received.  PSK31 best practices indicate that messages be transmitted mostly in lower case to achieve better message content throughput.

Regards,
Brian
VE3IBW