Currently, initial focus when editing a tiddler is automatically placed in the 'title' field (for new tiddlers) or the 'text' field (for existing tiddlers).
However, when using templates that contain custom fields, it can be desirable to specify which field should receive the initial focus (there might not even BE a 'text' field in the template!)
http://www.TiddlyTools.com/#CoreTweaks addresses this by adding an option: txtEditorFocus, which contains the field name of the field that should receive the initial focus. Depending upon the use-case, this value can be set as an individual user preference or defined by the document author as a 'sticky' setting by using a systemConfig tiddler to assign a value on document startup.