Look in the Electrical Characteristics section of the PIC data sheet. Look under Input
High Voltage. For a Schmitt Trigger type input you'll see something like 0.8 Vdd under
the Min column with Vdd under the MAX column.

What this means is; Minimum input that will be recognized as a logic 1 would be about
0.8 times Vdd. The MAX input level of course should not exceed Vdd.

Now look a bit farther to the right under Conditions. If this states "for entire Vdd range"
then this means 0.8 times whatever voltage you're operating at, that is within the spec
for Vdd, would be what you use for the input threshold calculation.

I.E. if the PIC you're using can operate from say 4V to 5.5V, then you would use whatever
Vdd is to figure this out.

Let's assume you're using a 4V supply. 0.8 * 4V = 3.2V. This would be the minimum input
level you need on the ST input to be recognized as a solid logic 1. This is the logic 1 input
threshold level.

Now look at the TTL input requirement. It may look something like this. Under the Min
column it shows 2. Under the Max column it shows Vdd. To the right it may be something
like 4.5V ≤ VDD ≤ 5.5V

2V is the minimum input level that will be recognized as a logic 1. Vdd is of course the MAX.

The 4.5V ≤ VDD ≤ 5.5V means (if Vdd is greater than or equal to 4.5V and less than or
equal to 5.5V), then you're operating within spec.

Output levels are also listed just below this section. A pin outputting a logic 1 will normally
be Vdd-0.7V, which just means you can expect a drop of around 0.7V due to the output
driver. If you're operating at 5V, then 5V-0.7V is what you can expect the output pin to
deliver to the load.

Can I use a ST pin for TXmit without any problems?
Yep. ST only refers to input pins. The DC Characteristics section of the data sheet will
show what you can expect for a drop in the output driver. Normally it's around Vdd-0.7,
but it never hurts to check.