describing a block diagram // How do I describe multiplying a signal through multiple branches formally?
What I would do is:


Add $m$ multiplier blocks, using a circle with a $\times$ inside
Each multiplier would have inputs $X$ and $Y_k$
Each multiplier would have output $f_k$
Be consistent in the use of bold typeface, uppercase and lowercase


Then I would say something like "The system has input $X$ and outputs $f_k=XY_k$, for $k = 0,\ldots,m$. In the diagram, $\times$ stands for signal-matrix multiplication, which is defined as follows...".