Both a lanyard and a self-retracting lifeline (SRL) connect a harness to an anchor, and both are sold as fall arrest equipment. They are not interchangeable, because they behave differently in the moments after a fall begins. Four factors decide which one suits a given job.
Clearance below the work position
This is the factor that most often decides it. A fall arrest system needs enough distance between the worker and the nearest obstruction for the fall to be arrested safely. A lanyard adds the length of the lanyard plus a deceleration distance plus the worker's body length before anything else is counted; an SRL adds far less, because the line locks after a short extension. Where there is little clearance below the work position, an SRL is normally the one option that fits.
Height of the anchor relative to the worker
An SRL performs at its best when the anchor is overhead, so the line pays out and retracts vertically. Anchor an SRL below the worker, at foot level, and a fall can develop the same long, high-force loading that a lanyard on a low anchor produces. Lanyards tolerate some off-vertical loading better in practical terms, but neither system is a licence to anchor below the worker. Anchor height is a design decision, not a detail.
Work radius and movement
A lanyard restricts a worker to a radius set by its length, which is sometimes what is wanted: it keeps the worker inside a defined zone. An SRL gives a larger working radius and follows the worker in and out, which suits tasks that move along a runway, a ladder or a large structure. If the job involves repeated in-and-out movement across a wide area, an SRL reduces drag and the temptation to disconnect.
Rescue and retrieval
Plan the rescue before choosing the equipment. A worker suspended in a lanyard after a fall needs to be reached and recovered; an SRL, or an SRL combined with a rescue winch, can allow a controlled lift or lower that a lanyard cannot. Wherever a fall is possible, the question is not only how to stop it but how the person is brought down, and the equipment has to be chosen with that answer in hand.
A monitored system records which one you used. When harness use is monitored, the sensor state confirms that a connector was attached, when, and for how long. That record turns a selection decision into something a supervisor can verify on the shift, rather than something that is asserted in a briefing.