Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 323

The maximum burst during continuous shooting is lower.
With ISO 12800 or "H" (equivalent to ISO 25600), the maximum burst
for continuous shooting will greatly decrease (p.91).
Set [z3: High ISO speed NR] to [Standard/Low/Disable]. If it is set
to [High] or if [Multi Shot Noise Reduction] is set, the maximum burst
during continuous shooting will greatly decrease (p.124).
With [Chromatic aberration: Enable], the maximum burst for
continuous shooting will greatly decrease (p.128).
During WB bracketing, the maximum burst for continuous shooting will
decrease (p.138).
If you shoot something that has fine detail (such as a field of grass),
the file size will be larger and the actual maximum burst may be lower
than the number mentioned on page 87.
ISO 100 cannot be set.
Under [54: Custom Functions (C.Fn)], if [3: Highlight tone priority]
is set to [1: Enable], ISO 100 cannot be set. If [0: Disable] is set, ISO
100 can be set (p.295). This also applies to movie shooting (p.174).
ISO speed [H] (ISO 25600) cannot be set.
Under [54: Custom Functions (C.Fn)], if [3: Highlight tone priority]
is set to [1: Enable], the [H] ISO speed (25600) cannot be selected
even when [2: ISO expansion] is set to [1: On]. If [0: Disable] is set
for [3: Highlight tone priority], [H] can be set (p.294).
The Auto Lighting Optimizer cannot be set.
Under [54: Custom Functions (C.Fn)], if [3: Highlight tone priority]
is set to [1: Enable], the Auto Lighting Optimizer cannot be set. If [0:
Disable] is set, the Auto Lighting Optimizer can be set (p.295).
Troubleshooting Guide
323