Hi,
I would get a dial like this :

A. Section one
1. Subsection one

With the code below, I get :

A. Section one
A. A. Subsection one

\setupsection
  [section]
  [conversion=Character]


\setuplabeltext
  [section={{},{.}},
   subsection={{},{.}}]


\setupsection
  [subsection]
  [previousnumber=no]

\setupbodyfont[pagella,11pt]

\starttext
\section{Définitions}
\subsection{Approche de la loi binomiale}

\stoptext

Why ?
Regards,
Fabrice