Once more than one settlement or outpost has been established on Mars, inter-settlement commerce becomes a possibility. Settlements can trade excess goods, rather than using valuable space for storage or recycling/wasting the goods. Once stable commerce has been established, some settlements may be able to specialize in a particular area, such as mining, manufacturing, or agriculture.
Trade involving physical goods necessitates a transport infrastructure. Initial transport between young settlements may involve rovers. Larger settlements may be able to invest in a railroad or similar dedicated transportation method.
Tourism is likely to become a type of inter settlement activity. Variations in development, local geographic attractions and settlement organisation will all create differences that will generate interest and inter settlement displacements. Settlements will develop distinctive attractions through historical divergences or deliberate decisions.