Help+Manual's table formatting functions are powerful but they do have some limitations. For example, you can only set a background picture for the entire table. Merging and splitting cells in a single table can also place limitations on column width adjustments.
Some of these problems can be solved by using nested tables, i.e. by inserting a second table inside the first table. Then you can apply different table properties to the second table, which can be a single cell, row or column in the "parent" table.
Temporarily apply thick colored borders to your nested tables so that you can see where they are while you are working with them. You can do this most easily by defining a table style and applying it temporarily while you are working.
Inserting a nested table is just like inserting any other table. The only difference is where you insert it:
1.Click inside the cell of the table where you want to insert another table.
2.Use any of the standard methods for inserting a new table.
The nested table is a fully independent table in its own right. You can use all the normal table formatting and editing functions on it, including applying a style to its "table paragraph". You just need to keep track of where your nested tables are, which can be tricky.
Don't go overboard with nested tables. It's tempting to use them but it can get too complicated very quickly!
Page breaks in PDF and DOCX output and printed manuals may not work correctly inside table rows containing nested tables, particularly if the tables contain merged cells.
Table Properties (Reference)
How table sizing works (Reference)