Select Append from the Block menu to save the selected portion of the file you are editing into another file. If the other file already exists, the selection is added to the end of that file. If the file does not yet exist, it is created. Use Block|Write if you want to overwrite the file if it exists.
If you do not specify an extension for the filename then Block|Append adds an extension depending on the file type you selected in the dialog box. This follows the same rules as File|Save As.