sin/cos periods per revolution, means you get an output signal like depicted X 1024 times = 1 revolution of encoder, or vice versa the depicted signal trace equals to 1/1024-th rev's movement.
If you transform sin/cos signals into quadrature pulses, you then get 1024 pulses per revolution.
total number of steps, 4096 revolutions X 1024 sin/cos periods per revolution X 32 possibly the extrapolated positions within one sin/cos period, this number depends on ADC and used MCU/DSP.
total number of steps, 4096 revolutions X 32768 steps/turn
Are the "REFSIN" and "REFCOS" signals -SIN and -Cos signals?
Yes. See the Hiperface documentation.
What is the difference between "sine/cosine periods per revolution"
and "number of the absolute ascertainable revolutions?"
sin/cos periods determine the position within one revolution, meanwhile there is yet another type of encoder embedded for multi turn. Usually a gearbox with a coded disk and hall sensors that do measure number of revolutions, similar like water meter.
Picture taken here
Some encoders use just a built in battery to store the absolute position, without a coded disk - in your case that would make sense