A bow knot and a single knot
A bow made on top of a single knot: The above only refers to [a double knot made of] two knots one of top of the other, however by one knot and a bow on top of it there is no doubt at all [that it is not considered] a professional knot and it is thus allowed to tie and untie in any case that it is not designated to last at all. [See Q&A]
May one make a bow on top of a bow?  Yes, it may be done even with intent to never undo.
Making a single knot: A single knot without a bow on top of it, is not considered a knot at all and is allowed to be tied and untied even if made to last forever. [See Q&A]
[However] all this refers to tying two things together, however if one makes a knot using the single end of a rope or thread or string then it has the same law as a double knot for all matters.