What is the difference between a ChemDraw iOS caption and an atom label?
A caption is just a block of text, with no chemical significance. Captions are simply annotations applied to drawings. An atom label does have chemical data associated with it, can have bonds attached, and is interpreted by the chemical analysis tools. The text tool is intended solely for creating captions. If you create a caption H2O, you cannot add bonds or calculate its MW for example. It is purely a piece of text. By contrast, if you create an H2O molecule using the periodic table tool, that is a valid chemical structure and can be analyzed and further modified. Note that the text tool cannot be used to modify atom labels, unlike the desktop version. Atoms can only be modified using the tools in the Atom toolbar.