Similar to the answer provided at how to represent cross and tick in itemize bullets, except this is for enumerate.
If you are using the enumitem package you can use \setlist[enumerate,<n>]{label={DAS.\arabic*}} to set the enumeration label for the depth of <n>:

\documentclass{article}
\usepackage{enumitem}
\setlist[enumerate,1]{label={DAS.\arabic*}}
\setlist[enumerate,2]{label={BOOT.\arabic*}}
\begin{document}
\begin{enumerate}
\item foo 1
\item foo 2
\begin{enumerate}
\item bar 1
\item bar 2
\end{enumerate}
\end{enumerate}
\end{document}
Alternatively, you can also do it on a per list basis:
\documentclass{article}
\usepackage{enumitem}
\begin{document}
\begin{enumerate}[label={DAS.\arabic*}]
\item foo 1
\item foo 2
\begin{enumerate}[label={BOOT.\arabic*}]
\item bar 1
\item bar 2
\end{enumerate}
\end{enumerate}
\end{document}
A third option would be to define your own list type:
\documentclass{article}
\usepackage{enumitem}
\newlist{DasList}{enumerate}{1}
\setlist[DasList,1]{label={DAS.\arabic*}}
\newlist{BootList}{enumerate}{1}
\setlist[BootList,1]{label={BOOT.\arabic*}}
\begin{document}
\begin{DasList}
\item foo 1
\item foo 2
\begin{BootList}
\item bar 1
\item bar 2
\end{BootList}
\end{DasList}
\end{document}
Without the enumitem package you use \renewcommand{\labelenum<n>}{DAS.\arabic{enum<n>}} where <n> is a roman numeral (i, i, iii, or iv) representing the nesting depth of enumerate.
The following yields a similar result to above:
\documentclass{article}
\usepackage{amsfonts}
\renewcommand{\labelenumi}{DAS.\arabic{enumi}}
\renewcommand{\labelenumii}{BOOT.\arabic{enumii}}
\begin{document}
\begin{enumerate}
\item foo 1
\item foo 2
\begin{enumerate}
\item bar 1
\item bar 2
\end{enumerate}
\end{enumerate}
\end{document}