Declaration: TParticles.MarkedAsSelected [PartIx: integer]: boolean;
The array property MarkedAsSelected contains the "Selected" flag of the particle editor (i.e. it reflects the state of the checkbox in the "PID" column of the particle list).

The parameter PartIx is the particle index (range 1..NParticles).

Hint: In order to access the particles currently loaded in the particle editor you can use the pre-declared global variable PEData.