Saks Off 5th, the discount sister chain to posh Saks Fifth Avenue, will open a 40,000-square-foot store next fall at Congressional Plaza on Rockville Pike. This is big news for fans of the New York-based company, who’ve been trekking to Leesburg, Arundel Mills and Potomac Mills for their Fifth Avenue treasure hunts. Off 5th is something like a Nordstrom Rack for Saks, carrying leftover goods from its sister store mixed with designer items and merchandise created specifically for its own stores. At Congressional, Off 5th will take the storefront formerly occupied by Tiara Galleries as well as a portion of the office space that currently houses landlord Federal Realty. Off 5th should be a good fit for Congressional’s increasingly luxury-oriented tenant mix, which includes Last Call Studio by Neiman Marcus, White House/Black Market, The Container Store and gourmet grocer The Fresh Market.  A beer and wine store recently joined the mix, and Chop’t is set to move in this spring.