MAPC2MAPC Help pages : Cache
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From version 518, map tiles downloaded from the Internet are held in a cache. This improves access speed when scrolling around the map or zooming in and out.

By default the cache is cleared after each session but a this can be changed with a checkbox on Edit>Preferences and Settings General Tab. Then the cache is preserved between sessions. Each time the program starts, it displays a summary of the cache contents by number of days since last access. The cache will grow indefinitely - but hitting F2 will clear the oldest day's files.