Sorry but that still is not specific enough. When you say "+ icon" what type of device are you using? Is the + icon appearing at the top of the task list, or is it a widget or shortcut?
To try to answer your question, some places like the rapid task entry box in windows let you select a folder where the new task will be placed. Some places like the add task shortcut on android or the task by email facility default to the inbox. Otherwise, the + icon creates a new task as a sibling to the currently selected task, placing it in the same folder as the currently selected task.