How Designers Can Utilize component Reference Designators during PCB design?

During layout, reference designators are excellent guides for nearby placement of components according to the schematic. Cross-probing between the schematic and board layout allows designers to quickly group components proximally according to the reference designators used in the schematic pages to minimize the impedance pathways between them. From here, designers can optimize the layout according to best practices, manufacturers’ datasheets, etc., for finer placement adjustments. 

Remember that the reference designator is the most immediate and reliable method of aligning the BOM, schematic, and board file. Any alterations to the reference designators arising from revisions/updates must be present in all three locations to ensure the board's documentation remains in lockstep. These changes should always be forward annotations from the updated BOM to the schematic before netlisting the changes into the board file. This workflow has the BOM remain the controlling document for the project where any assembly documentation issues (missing or incorrect component/reference designator) are by version history in the BOM or a digital paper trail.

However, certain designs may demand a fixed reference designator, such as that of a connector. The idea is that a component may be common to multiple sub-assemblies within a larger system: to align component legends between multiple sub-systems, it’s necessary to “force” the reference designator(s) to a desired value. This commonality will constrain the reference designator assignment in some fashion, but the general rules for placement, legibility, etc., are equally applicable.
Component reference designators are essential to troubleshooting assembly issues, and with assembly density tending to increase over time, it’s worthwhile to take the necessary steps to verify this layer remains useful as a quick reference. While arranging the reference designators is usually low on the list of designer worries, its difficulty and importance continue to increase with HDI designs. 

Know more:Component Reference Designators