The goal of both logical and physical architecture specifications is to define and document the logical and physical components of a system, respectively, in order to provide clarity around how those component elements relate to one another. The artifacts resulting from either effort could be text documentation, or diagrams, and both have their own advantages and drawbacks.
Text documentation is usually quicker to produce, but unless there is some sort of architectural documentation standard that can be applied, the formats can (and probably will) vary from one system team to another, at a minimum. That sort of variance can make it difficult for the resulting artifacts to be understandable outside the team that it originated with. If there is not a lot of movement of developers between teams, or a significant influx of new developers to...