1 Answer

It means the governance is by Corporation law---seems that 1871 was the year that the corporate body of the United States of America was formed.----(Municipal Law) -- 

a " Corporation" 
with a legislature was established, 
with all the apparatus of a distinct government 
created (Incorporated) by (Presidential) Legislative Act, February 21, 1871, Forty-first Congress, Session III, Chapter 62, page 419 

On June 20, 1874, the President with advice of Senate abolished and replaced the 1871 government with a commission consisting of three persons. 18 Stat. at L. 116, chap. 337 

A subsequent act approved June 11, 1878 (20 Stat. at L. 102, chap. 180) was enacted stating that the District of Columbia should 'remain and continue a municipal corporation,' as provided in 2 of the Revised Statutes relating to said District 
(brought forward from the act of 1871) 

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