A Field on the Data Model can be Linked, Driven or Hooked. These are different ways of specifying the type of control a system can have over fields it is influencing. In the Scene Inspector, linked fields are highlighted blue, and driven or hooked fields are highlighted purple. There you can also easily find the source of a linked / driven field by clicking on the hamburger menu button on the left side of the field, and choosing "open link / drive source". You can also break the linkage by choosing "break link / drive", which will set the active link / drive / hook reference to null.
Links
Links are exclusive references to fields. There can only be a single thing linking a given target field at the same time. This is often used when systems need some sort of exclusive control over the target and make sure no other copy of that system will operate on it. Linked fields are highlighted blue in the Scene Inspector.
Drives
Drives are an extension of links that on top of ensuring exclusive control also give the drive source control over the value of the driven field. This excludes the field from the implicit synchronization by the Data Model, meanings its value gets locally computed by every client, and changes to it are not networked. Driven fields are highlighted purple in the Scene Inspector.
Drives are the fundamental building block that allows creating Local Systems (systems that aren't implicitly synchronized). By utilizing drives, fields can have different values for different users, which is required for things like culling systems to function. Drives can also be used to reduce network impact of systems, by locally computing values on every client, instead of computing them on one client and implicitly synchronizing them to all other clients by writing them to a non-driven field on the Data Model.
Hooks
Hooks are an extension of drives that allow defining custom behavior for writing to a driven value. They do this by signaling any attempted write to the field, and returning what the value being written was. This allows systems to react to external changes, for example to create "writeback" behaviors. Just like normal drives, hooked fields are highlighted purple in the Scene Inspector.
Components can have boolean values (often called write back or reverse mapping) that can switch the field hook behavior of the fields they drive. A couple of examples of the many components that can do this are ValueCopy and LinearMapper1D. For these kinds of components, they will allow sending data in the reverse direction when writing to the hooked value. For example, if 10 was written to the field hook's source value, and this causes the hooked value to become 20; Writing the value 20 to the hooked value will make the source automatically set to 10. This only works if the component has its write back switch enabled.
This is useful for when you want to make 2 containers synchronize their capacity in terms of volume, or to synchronize sliders or visuals in a cross link fashion.