\begin{tikzpicture}
\umlemptyclass{A}
\umlemptyclass[x=4]{B}
\umluniassoc[arg=bb, mult=1, pos=0.95, align=right, name=uniassoc]{A}{B}
\umlnote[y=-3, geometry=|-|, anchor1=70, arm=0.5cm]{uniassoc-1}{I am a note attached to the uniassoc relation}
\end{tikzpicture}