Confirm what turf is actually present
Do not rely only on colour or a distant photo.
Leaf width, tip shape, runners, growth habit and any known variety help. Kikuyu, zoysia and other turf can also be mixed into an old lawn, changing what can be applied over the whole area.
A selective herbicide has a defined crop list
The registered label identifies the turf and weeds for which a product may be used.
A broadleaf product approved for couch is not automatically appropriate for buffalo. Grassy-weed and sedge control can be even more constrained. The operator matches turf, target, condition and label rather than treating “lawn” as one plant.
Authoritative reference: APVMA guidance on choosing registered chemical products.
Healthy and stressed turf can respond differently
Heat, drought, waterlogging, disease, scalping and recent treatments can make an application unsuitable.
A thin buffalo lawn under shade or a drought-stressed couch lawn may need the underlying condition addressed first. Postponing selective weed control can protect the turf you are trying to keep.
Mixed areas may need targeted treatment
If no suitable whole-area option exists, weeds may need spot treatment or another control method.
That is preferable to risking one desired turf species for the sake of a fast blanket application. Close photos of both the weed and the surrounding turf can help before the visit.