What is the output type of the 74AHC594 IC?
I didn't find the answer in this datasheet. There is stated in some datasheets for the 74594 IC from other manufacturers that the output type is buffered. What does that mean?
It is a CMOS output. The specifications are in table 6 in the datasheet:
There is stated in some datasheets for the 74594 IC from other manufacturers that the output type is buffered. What does that mean?
The specifications of 74594 aren't relevant. Those are for the original 74xx logic family, which was a TTL family. Your part is in the AHC family, which is CMOS.
If the part had buffered outputs, that meant it was able to drive or sink more output current than un-buffered parts in the same family.
Note: Your datasheet also covers 74AHCT594, which is a CMOS part with TTL-compatible inputs. If you don't want TTL compatible inputs, don't use this part and don't worry about its specs.
Usually, the transistors that are used to perform the actual logic functions on a CMOS logic IC are relatively small. However, these small transistors are not able to quickly change the logic state of the device's physical pin and any wiring capacitance associated with it. So, the internal logic signals are buffered by passing them through a series of inverters where the transistors in the inverters gradually increase in size. Eventually you have an inverter whose transistors are large enough to drive the external pin, so you connect that inverter output to the outside world. As long as you have an even number of inverters in series the internal logic state will be correctly driven onto the external pin.
I would assume that almost any modern logic device would have buffered outputs. In ancient times there were some CD4000 series devices that were available in both buffered and unbuffered versions.