The Datum Feature Symbol function creates reference element symbols. These are required, for example, for referencing tolerances.
To be able to assign a tolerance to the model in the next step, you first need a reference symbol. To do this, switch to the "Top" view and click Datum Feature Symbol
so that the Datum Feature Symbol dialog box opens.
The center line of the upper cylinder should be given the reference symbol "A". The desired letter for the reference can be entered under Datum Identifier. Then click on the Center Line of the upper cylinder, hold down the LMB and drag your icon to the desired position. Clicking again will finally discard it. You can reduce the length of the resulting arrow under Leader->Style->Stub Length.
The result should correspond to the figure (see figure "Top with DFS").