One aspect of the president's relationship with congress is that the
congress must approve any request from the president for military funding
president must give an address regularly to congress on the state of the union
congress must approve any of the president's appointments with a new law
president must give a commission to the newly elected members of congress

A major aspect of the relationship between the President and Congress and is also "one aspect of their relationship" is that Congress must approve any request from the president for military funding. As Commander and Chief the President can request specific funds for the military, but Congress still retains the power of the purse and controls whether or not it will grant the President their funding request.