The 49ers are moving to a 43 under defense with three linebackers. A weak side linebacker or a WILL that plays on the second level, a middle linebacker or MIKE that also plays on the second level and a strong side linebacker or SAM that usually lines up on the line of scrimmage playing the 7-technique outside of the tight end. The SAM can also line up in the secondary at times.
Both Ahmad Brooks and Aaron Lynch have experience playing left outside linebacker in the past which appears to be similar but, given Lynch’s weight and injury issues along with last year’s suspension and Brooks’ abilities seem to be greatly declining with his age, it would not surprise me if the 49ers brought someone else into camp to compete for the position.