# Using appendix package with subfiles

I'm trying to add some appendices to my thesis (using the book class), which I've done so far using subfiles, that way I don't need to compile the entire mainfile every time and I do avoid switching between the main file and the single file to compile the second one. But once I had the need to add an appendix I thought of doing the same as before just with the need of Appendix package, so far I have something like this inside the document (Main File)

\begin{appendices}
\subfile{ File A}
\end{appendices}


And in the single file I have the following:

\documentclass[../MainFile.tex]{subfiles}
\begin{document}

\chapter{Lebesgue measure}
\begin{Def}
Some definition
\end{Def}

\end{document}


Seems the problem is whenever I start the individual file with chapter, many error come, but if instead I use section there's no problem, but only shows something like

.1 Lebesgue Measure

Instead of what I'm looking for, which is

Appendix A

Lebesgue Measure

PD. Is there any way so that the definitions, theorems and so on stop numbering, i.e. , The last definition before the appendix was 90, and after the appendix continuous with definition 100. Can I have just "Definition" ? For that I have the statements:

\newtheorem{Def}{{Definition}}