In this paper we extend the study of linear spaces of upper triangular
matrices endowed with the flag-rank metric. Such metric spaces are isometric to
certain spaces of degenerate flags and have been suggested as suitable
framework for network coding. In this setting we provide a Singleton-like bound
which relates the parameters of a flag-rank-metric code. This allows us to
introduce the family of maximum flag-rank distance codes, that are
flag-rank-metric codes meeting the Singleton-like bound with equality. Finally,
we provide several constructions of maximum flag-rank distance codes