As of Cinema 4D 2025.1, it is also possible to attach custom properties to the particles, which can then be queried again in Conditions, in Math or Data Mapper modifiers, for example, or offset against other properties of the particles. The Flock Modifier offers two properties for storage.

To use the corresponding storage, activate the option to the right of its name. A new custom property whose data type is compatible with the output value is automatically created and entered. These automatically generated custom properties are stored within the Scene Settings and their Particle Simulation settings and get listed in the Automatic Custom Properties section.
However, you can also go to the group for Manual Custom Properties and create your own custom properties there. You will find an arrow button behind the respective name field of the Output properties, which can be used to open a selection list of the available custom properties. These lists are automatically filtered to display only the custom properties of the simulation scene whose data types are compatible with the Output value.

You can find out more about the manual creation of custom properties for saving and processing particle properties in the section on Particle Simulation Settings within the Scene Settings.

When using the following outputs, please note that the setting for Neighbour Search is also evaluated for their determination. In simulations with a large number of particles, an incorrect Particle ID or an insufficient Neighbour Count may therefore be output in the Fixed or Dynamic modes if too low a value is used for Maximum Checks.

Closest Particle ID

The Integer index number of the nearest neighboring particle is output here. If no particle can be found within the search radius, the value -1 is displayed instead. Activating the Closest Particle ID option automatically creates a custom property with a suitable Index data type. Alternatively, you can click on the arrow button next to the name field to select one of your manually created custom properties that also uses this data type.

Neighbour Count

This integer value specifies the number of neighboring particles that are within the search radius. Activating the Neighbour Count option automatically creates a custom property with a suitable Integer data type. Alternatively, you can click on the arrow button next to the name field to select one of your manually created user properties that also uses this data type.

The following video shows an example of a possible use. There, the number of particle neighbors in a Color Mapper is used to color the particles.