Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 89

About 1
1 is the raw image data before it is made into 73 or other images.
Although 1 images require software such as Digital Photo
Professional (provided, p.350) so that they can be displayed on the
computer, they also offer flexibility for image adjustments possible only
with 1. 1 is effective when you want to precisely adjust the image
yourself or shoot an important subject.
About 1+73
1+73 records both a 1 and 73 image with a single shot. The
two images are saved to the card simultaneously. The two images will
be saved in the same folder with the same file numbers (file extension
.JPG for JPEG and .CR2 for RAW). 73 images can be viewed or
printed even with a computer that does not have the software provided
with the camera installed.
Commercially-available software may not be able to display RAW images.
Using the provided software is recommended.
3 Setting the Image-Recording Quality
1 image
73 image
File number
0001 . CR2
0001 . JPG
File extension
89