MWE

```
\documentclass[a4paper]{article}
\usepackage{dcolumn}
\usepackage{multirow}
\usepackage{lipsum}

\begin{document}

\newcolumntype{d}[1]{D{.}{.}{#1}}
\begin{tabular}{l|c|r|d{2}|d{1}|d{0}|d{-2}|}
aaaaaaa     & bbbbbbbbbbbbbb & cccccc        & ddddddd & eeeeee & ffffff & gg \\
    abcdefg      & abcdefg        & abcde         &  123.45 & 123.45 & 123.45 & 123.45 \\
    abcdef       & abcdef         & abcd          &  123.4  & 123.4  & 123.4  & 123.4 \\
    abcde         & abcde          & abcde         &  23.45  & 23.45  & 23.45  & 23.45 \\
    abcd      & abcd           & abcd          &  3.45   & 3.45   & 3.45   & 3.45 \\
    abc            & abc            & abc           &  23.4   & 123.45 & 23.45  & 123.4 \\
    \end{tabular}
\lipsum[1]

\begin{tabular}{l|c|r|d{2}|d{1}|d{0}|d{-2}|}
aaaaaaa     & bbbbbbbbbbbbbb & cccccc        & ddddddd & eeeeee & ffffff & gg \\

abcdefg     &
\multirow{4}{*}{\begin{tabular}{r} zzzzz\\ zzzz\\ zzz\\ zz\\ z\\ \end{tabular}}
                             & abcde         &  123.45 & 123.45 & 123.45 & 123.45 \\

abcdef  &    & abcd          &  123.4  & 123.4  & 123.4  & 123.4 \\
    abcde     &    & abcde         &  23.45  & 23.45  & 23.45  & 23.45 \\
    abcd  &    & abcd          &  3.45   & 3.45   & 3.45   & 3.45 \\
    abc        &    & abc           &  23.4   & 123.45 & 23.45  & 123.4 \\
    \end{tabular}

\end{document}

```

![enter image description here](https://i.stack.imgur.com/29mVi.png)

Questions

* what is the difference between `{2}, d{1}, d{0}, d{-2}`? According to manual it number of decimal places, but this option does not cut the numbers, doesn't change 23.45 into 23.4 (for `d{1}`), what for is this option?
* in manual is `\newcolumntype{,}{D{.}{.}{2}}` but doesn't work (Error: Illegal pream-token (d): `c' used.) why and what's the difference?
* look at second table, second col, how to do it without nesting tables? how to align column to right and later place in the center?