Two letters, one design decision — The system earthing arrangement is written as two or three letters. It is not a label - it fixes the fault loop, and with it the current, the device that clears it and the time allowed.
What the letters mean — So TN-C-S is not a third system: it is a TN system that is C at the beginning and S after the split point.
TN-S — One point of the supply earthed at the source; exposed-conductive-parts tied back to that point by a separate PE. The fault loop is metallic, so the fault current is high and an overcurrent device can clear it.
TN-C-S — Neutral and protective functions share one PEN conductor, then split. No switching or isolating device may be inserted in a PEN conductor, and downstream of an RCD a PEN shall not be used - the PE connection is made on the source side.
TT — The installation has its own electrode, so the loop closes through soil and the fault current is modest. IEC 60364-4-41, 411.5.2: RCDs are generally used, and 411.5.3 requires R_A x IΔn ≤ 50 V.
IT — The first fault gives a small current and does not have to disconnect anything, provided R_A x I_d ≤ 50 V (411.6.2). It has to be found and cleared, because the second fault on another phase is the dangerous one.
What follows from the choice — Times from IEC 60364-4-41, Table 41.1, for 120 V < U0 ≤ 230 V. Distribution circuits are allowed 5 s in TN and 1 s in TT.
What to check on a drawing — Reference: IEC 60364-1:2005, 312.2 and IEC 60364-4-41:2005+AMD1:2017, 411.4 to 411.6.