\definedescription[Mat]
  [headstyle=bi,
   text={Foo},alternative=serried,width_=fit,indenting_={yes,medium},indenting_=no,align_=left,indentnext_=no,hang_=none,
  ]

\starttext

  \startMat
    \startitemize[packed,][]
      \starttabulate[|l|l|l|]
        \NI Aaa \NC Aa \NC\AR
        \NI Bbb \NC Bb \NC\AR
      \stoptabulate
    \stopitemize
  \stopMat

  \startMat
    \startitemize[packed_,joinedup_][afterhead=,]
      \head Ttt \par
        Tt

      \head Uuu \par
        Uu
    \stopitemize
  \stopMat

\stoptext