You could place SMT components right next to each other so that there is only a minimal gap between them, however doing this can affect assembly as one component can affect another, heatwise or knocking it during placement etc.
To prevent assembly problems an outline is placed around the components, extending beyond the body and the pads, this is called (in IPC) the courtyard. In CADSTAR it is called the placement outline because it is used during placement of the components.
An old rule of thumb was that there should be 1mm between pads of different components, this was achieved by using a courtyard that extended beyond the pads by 0.5mm, so placing 2 next to each other achieved 1mm spacing.
This would be the courtyard manufacturing zone (it includes all spacings and gives the best assembly placement - not always the best electrical placement spacing).
However things have moved on since then and although 1mm spacing is an optimum, it is rarely achievable these days as boards are more dense, smaller board - same no of components. The IPC spec has 3 levels of density & you can pick the one you want to use, the spec also calculates the courtyard depending upon the size of the component etc likely using some algorithm I can't do on my fingers
(I cannot remember exactly what 7351 says and do not have it to hand but ISTR it does suggest something).
I wouldn't get hung up on it, use the IPC spec for the courtyard and if the component is not in the IPC spec then use 0.5mm space around the pads - if that turns out too big reduce it until you can get it to fit without assembly problems. (The felxible approach
Some people have their courtyard right on the edge of the pads, personally I think this is just asking for assembly problems however what one placement machine can do, others cannot.
IN CADSTAR, not having a placement outline or having one that is smaller than the area of the pads prevents a proper DRC check
as this outline is used for the DRC also. I.E. Component to component checks etc.