This might help. Here are the three most common types of system.
Y-Plan (One 3-Port Motorised Valve)
View attachment 175566
S-Plan (Two or More 2-Port Motorised Valves)
View attachment 175568
Gravity Circulation Hot Water & Pumped Central Heating (No motorised valves, hot water not pumped)
View attachment 175570
There are other possible configurations, but they are much less common.