Some pads on a PCB are marked in clusters and I can't understand which one is which
sorry for the poor drawing. i hope you can trace them.
I don't know the technical terminology for this. We do this whenever there is no space to place the reference designator at the same time it can't be dropped all together too. Then Reference designator will be grouped together but at some other point in the PCB where there is abundant space.
Care will be taken to see that the cluster will be exact replica of how it could have been in it's ideal place. The orientation of the components will be represented by the orientation of the reference designator.
Naming the clusters help in locating them easily. When it is easier we also have simply drawn lines from the component cluster till the label cluster.
If somebody knows the terminlogies please add.  remote designators thanks to @RnDMonkey
Test points for Production
During production of the PCBs (say 1000 s of them) the testing will b done by automated machines. The machines do not need any text. They work by knowing the position of components. Here, they use those big round test points.  Through these test points the machine can measure resistances, capacitances, inductances as well as voltages.