When you create a sketch, the sketch (.sec) will be added to your session memory. You can see this if you look at the object list (File>Manage Session> Object List). You can reuse the sketch in another part by performing the following steps:
Initiate a new sketch
Select sketch plane and orientation references
Select the File System command
Choose to retrieve a section from "In Session"
Choose the .sec file
Place the sketch - beware of custom scaling factors
If you close the file and erase memory, the sketch (.sec) will also be erased. Reopening the file that contains the sketch does not add the sketch (.sec) back into memory. So, if you erase memory, the steps from above no longer work. You would need to do one of the following:
Reuse the sketch in another part before erasing memory - using the steps from above
Save the sketch to a location that can be accessed later
***Use copy/paste instead*** - This is what I would recommend.