Most single-line text fields store multiple parallel strings of text, each with a different writing system so you can have translations of the string of text in different writing systems. Writing system identifiers are displayed on the left end of each line to indicate the writing system for that line when more than one writing system is used. Usually, you can configure writing systems for fields individually.
In some, not all, you can embed characters using other writing systems. For those that permit embedding, the writing system box on the Format toolbar or Writing Systems on the Format menu are available so you can select the writing system in which you will embed the text. (If the font supports it, you can use the Character Map to insert special characters, even in a line that does not support embedded text.)
For some, you can use character styles to change selectively the appearance of words in the text. (However, others do not permit any formatting, such as some Lexeme Form field, although you can apply styles when you configure the dictionary.)
Most allow you to configure which writing systems to display in that field using the menu button 
next to the field's label. The selections you make do not affect other fields.
The Title field in Notebook allows you to specify the writing system it uses, but only allows one writing system displayed at a time.
Other single-line text fields are set by the program (or by you, in the case of custom fields) to store only a single line of text using the default analysis or vernacular writing system. However, you may be permitted to embed text using other writing systems.