Let's say you want a three column table, but you want the top cell to spread across the entire first Row of the Table AND you want the first cell in each of the remaining rows to merge vertically, something like this:

We do this with Special Tokens we drop into Cells.  Let's do the first row first.  Make a Token somewhere at the beginning of your Flow.  Label it "Span 3" (without the quotes) and set its value to:

Now make a Table Point followed by a Row and a Cell Point.  We will ignore the colours, margins, etc and just focus on the structure, but to make it clear to see what's happening, set a Border Width on the Table to 1 (One) so we can see the edges of each cell.

Now in your first Cell Point, put the Span 3 token before your text content.  You don't need to make the next 2 cells in that Row - the Span tells it to take up the space of 3 Cells.

That's the first row sorted!

Now make 2 Token Points somewhere at the beginning of your Flow.  The first one should be labelled "Start Row Group" (no quotes) and its value should be set to:

The second one should be labelled "Join Row Group" (no quotes) and its value should be set to:

Navigate back to the Table (Search can help), select the last Cell Point, and add a new Row Point, and this time add 3 Cell Points.  As you might have guessed we are going to make a Row Group.  In your first of those 3 Cell Points, drop the "Start Row Group Token" in at the beginning of your content.  Then put whatever text into the next 2 Cells.

Next add another new Row Point with 3 Cell Points of its own.  The first of these Cell points wants ONLY a Join Row Group Token.  It doesn't need content, because it is going to give it's space to the Cell above it.  Then put whatever text into the next 2 Cells.

Repeat the previous step for one last Row.  And that's it!

This visual guide might help:
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Note that to get exactly the layout in the sketches, the Cell with the "Start Row Group" Token needs its Vertical Alignment set to Center, and all the Table Point needs its Cell Margins set to Minimum or Medium.  You might also need to experiment with Column Widths in the Table Cell.

Also note that while you should be able to combine these features to get even trickier layouts, but it can be easy to get confused especially when using Packs.  Just remember to think row-by-row, and consider building the table statically just to get the Tokens right before you introduce more complexity using Packs.