What are the default keys for marking out in a sequence?

The default keys for marking out in a sequence are the letter keys O and R. When using editing software, such as in a nonlinear editing environment, marking out is an essential function that allows the user to specify the end point of a selected portion of the video or audio. The "O" key is typically used for marking an out point to indicate where the selection ends, while the "R" key may be used in conjunction with other functions like ripple editing or inserting. This function is critical for precision in editing, enabling the editor to determine exactly where clips should start and stop in a sequence.

The other options do not standardly represent the keys for marking out. Hence, understanding the role of the O and R keys in this context helps clarify why they are the default settings for this specific function.