Object groups are not only objects that are simply bound together. They can be also used for setting of scrolling content. You can actually change the size of group frame so that the group content is larger than the frame, and the reader himself can then scroll freely through the content.
In a special case it is necessary to group together a single object. For example, you have a text that is longer, but you want it to appear inside of small window. So first you create a text box, e.g. 300 pixels wide and 1500 pixels tall and fill it with the text. Then group together the text frame using a Group Enter image description here button. Make the resulting group smaller so that the object is still 300 pixels wide, and 250 pixels tall for example. Next, select that the content will scroll vertically.
Reader then sees a large box 300 x 250 pixels on his iPad, and inside he may scroll the content using his finger. Do not forget to suggest readers that the box contains more text: it is ideal to reduce the group in a such way that it is obvious that the text continues below (e.g. the letters are cut off at the bottom), or consequently use the frame with information text (e.g. “Drag for more information “).
This element can also be used for large tables: if you have prepared them in Excel or even InDesign, save them as two images: one is static and will contain e.g. legend of lines while the other one will be scrollable and shall contain individual columns, e.g. left or right scrollable.
The fact that a group of objects includes content which may scroll, may be also identified by the color of edge: when the bottom edge is red than it means that more content continues below the edge and the object is scrollable in vertical direction. If the right margin is red then the horizontal scrolling is set for the object.