siunitx overfull hbox dependent on table-format

Question

There are a few similar questions (see here and here) regarding overfull \hboxes in siunitx tables but neither solves/explains the following. Consider this MWE:

\documentclass{article}

\usepackage{booktabs}
\usepackage{siunitx}

\begin{document}

\begin{table}[t!]%
\centering%
\sisetup{detect-none, mode=math, table-format=+1.1, table-auto-round}%
\begin{tabular}{ls[table-alignment=left]SSSS}%
\toprule%
A & \newton & 0.32 & 3.8 & -4.5 & -0.6 \\%
B & \newton & -0.12 & 1.1 & -1.6 & -1.0 \\%
C & \milli\metre & -2.45 & -7.0 & 2.3 & -4.3 \\%
\bottomrule%
\end{tabular}%
\end{table}%

\end{document}

When compiled with pdfLaTeX, overfull \hbox statements are present in the log file (these statements however are not picked up/flagged by TeXniCenter as warnings).

When the table-format option is altered to table-format=+2.1 the warnings are still present but when table-format=+3.1 the warnings suddenly disappear from the log file.

I have two questions:

1. Why does table-format=+1.1 or +2.1 produce overfull \hbox messages in the log file but not table-format=+3.1? Have I misunderstood something obvious in the siunitx documentation?
2. Why are the pdfLaTeX warnings not raised as LaTeX warnings? Is this something specific to TeXnicCenter or a more general behaviour?

Answer 1

The table-format key is used to set up how much space siunitx reserves in a table column for numbers. As such, it should reserve enough space for the largest entry in that column. The format is the same as a number, and the input is processed by the same code. As such, if you need to reserve space for a sign then using - rather than + is recommended as with standard settings + will be 'thrown away' during parsing. Thus +1.1 will (probably) only reserve space for one digit before and one digit after the decimal marker, with none for the sign.

You will see the warnings disappear if you allow enough space, even if it's the 'wrong kind'. Thus something like -2.1 will reserve (in most cases) the same space as +3.1, as the entra 'digit' and the sign are the same width. Thus over-specifying digits may well remove warnings, but at the cost of loosing proper control of what is really going on.

The overfull box warnings will be in the log and come from TeX, not LaTeX. How they are then presented depends on the editor you use.