If you want to insert nonbreaking spaces between two words you have to use the tilde (~). Non-breaking spaces are also useful to stop UDO and the other formats from breaking lines between two words.
Converting to an ASCII format UDO replaces this tildes by blanks. Converting to other formats UDO replaces this tildes by commands that have the same effect.
If you want to display a tilde you have to use !~ instead.
If you use a tilde inside an external link ((!url), (!xlink)) UDO won't convert it.