All backup operations (such as copying, purging, and so on) occur when you save changes to a model. If necessary, you can use backup files to roll back the latest changes to a model, restoring it to a previously saved state.
When you save a model, Revit makes a backup copy of the previous version of the model (that is, the model before the current save). This backup copy has the name <model_name>.<nnnn>.rvt, where <nnnn> is a 4-digit number indicating how many times the file has been saved. The backup file resides in the same folder as the model.
You can specify the maximum number of backup files that Revit saves. If the number of backup files exceeds the maximum, then Revit purges the oldest files.
For example, if the maximum is 3 backup files, and the folder contains 5 backup files, then Revit deletes the oldest 2 backup files.