Auxiliary files like .aux, .toc or e.g. .out etc. are files generated by LaTeX and packages which hold auxiliary information about the document. Specific information like sectioning titles or labels are written to these files. In following compiler runs this information is then read at the begin of the document so that there are globally available.

This feedback allows LaTeX to e.g. display the Table of Contents at the beginning while the required data (the sectioning titles) are not yet read.

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