What the terms mean
A single-split system connects one indoor unit to one outdoor unit. A multi-split connects several compatible indoor units to one outdoor unit. Each indoor unit can normally be controlled separately, but the shared outdoor equipment has an overall capacity and operating rules.
Single splits can be straightforward for one room or a staged project. Multi-splits can reduce the number of outdoor units where several rooms are involved, although pipe routes may be longer or more complex.
Capacity and simultaneous use
The number printed beside each indoor unit does not simply add up to unlimited simultaneous output. A multi-split design considers the connected combination and the outdoor unit capacity, together with which rooms are likely to run at the same time.
If one bedroom is used at night and a home office during the day, the pattern differs from several occupied bedrooms running together. The survey should use the actual household routine rather than assuming every unit has identical demand.
Installation and future decisions
Outdoor-unit location, pipe routes, access and future servicing can make one arrangement more practical than another. A single fault in shared outdoor equipment can also affect multiple indoor units, while separate systems provide more independence.
There is no universal winner. Ask for the reasoning behind the design, compatible model combinations and how later expansion would work before choosing.