Those are the areas for setting up of shopping areas. Big subdivisions are required to apportion some areas for such if they are to get approval for sub-division. If it's a residential you will need to set aside land for a market and school etc. Under strict zoning laws you cannot set up commercial building in the residential area. Syokimau started off that way clearly defined commercial areas and residential. Nowadays commercial buildings are coming up all over the place. Mavoko municipality used to enforce/regulate the building development but county govts are quite useless.

Some areas are designated as mixed use development so you can put residential and commercial in the same area and same building. e.g. Kirinyaga road

Kenyan land system is defined by the Constitution of Kenya, the Land Registration Act and the Land Act. It is classified into:

Public land - reserved for public use or environmental protection. It is administered and managed by National Land Commission on behalf of the people of Kenya.

Community land - it is held by communities on basis of ethnicity, culture or similar community interest.
Private land - this is land held by natural or legal persons. The Ministry of lands is tasked with the registration of any interest in private Land. It is classified into the following land tenure system;

Freehold land tenure system - it gives the holder absolute ownership of the land for life. A freehold title deed generally has no restrictions as to the use and occupation of the land. However there are some conditional freeholds which may restrict the use of land for agricultural uses only.

Leasehold land tenure system - this is the interest in land for a specific period of time subject to payment of land rent to the National government and land rates to the county governments. Once a lease expires the land reverts back to the owner or the leaseholder can apply for a renewal or extension of the lease.
https://eregulations.invest.go.ke/menu/117?l=en

I have seen some saccos and land selling companies selling "commercial plots" in freehold land areas, and the prices are always slightly higher than residential plots. What is the definition of a commercial plot in such a setting. Does it mean, for example, someone who buys one can put up a commercial skyscraper or mall there while an ordinary residential plot owner (under freehold) cannot? Thanks in advance wazuans.

I bought the so called "Commercial" sold by Ndatani near Newlight School in Kitengela but title deed is categorized the same as residential plots. In most cases, the so called commercial plots by land sellers are just a marketing gimmick (you pay more), and you would have to do a Change of User to commercial or Business-cum-residential plot.