The EXT has specification in the datasheet.
Trigger System
Level
EXT: ±0.6 V (Two channel series)
EXT/5: ±3 V(Two channel series)
Coupling Frequency Response (EXT, Two channel series)
DC: Passes all components of the signal
AC: Blocks DC components and attenuates signals below 20 Hz
LFRJ: Blocks the DC component and attenuates the low-frequency components below 7 kHz
HFRJ: Attenuates the high-frequency components above 160 kHz
Sensitivity
EXT(Two channel series): 200 mVpp DC– 10 MHz; 300 mVpp 10 MHz - BW frequency
EXT/5 (Two-channel series): 1 Vpp DC – 10 MHz; 1.5 Vpp 10 MHz -BW frequency
For example, if you want to use EXT for trigger, you should feed at least 200mVpp signal from DC (maybe they mean very very low frequency, not real DC) to 10MHz, or 300mVpp for 10MHz above, for stable trigger. You can feed no more than absolute maximum voltage (forgot where this specs are, should be the same as channel input) into this port, but out side ±0.6V range can not be triggered.
In theory, any waveform is OK, in practice, square wave will have the best trigger performance. The higher the slew rate is, the better.