Arduino timer ctc mode 4 vs 14
As described in the operation part of the datasheet section, OC1x is double-buffered; the code sets a shadow buffer to a given value and then at some point the hardware copies the shadow buffer to the register that controls operation. "Immediate" means that the buffer is completely transparent since the value is copied immediately.

The difference between modes 4 and 12 is that mode 4 allows the use of ICP1 for input capture and mode 12 allows the use of OC1A for waveform generation.