When the end grain is running between 30-60 degrees off parallel of the face of the board. Typically produces “straight grain” as opposed to flat sawn cathedrals and is easier to get a visually seamless glue line when edge gluing for width.
You will typically get sections of rift / straight grain closer to the outer edges of wider boards. Looking at the growth rings of the end grain of a log will tell you why that is.