Sequences, or regions of sequences, can be linked to external databases. For example an entire sequence entry or for when annotation tools are used to annotate proteins with domain or motif information (e.g. InterProScan available in MacVector 15). Very useful for when you want to view more detailed or updated information. Within the Genbank specification, which MacVector extensively uses, an external database entry can be stored in a /DB_XREF qualifier. This allows the database entry to be easily viewed. The Genbank (and Genpept) specification allow for many different databases to be accessed using this qualifier.
In MacVector the original database entry can easily be viewed in a web browser by selecting, then right clicking the feature entry in the Features tab and viewing the available DB_XREF entries. Selecting one will load it in your web browser.