に止(とど)まらずNot limited to, Not only...but also, Not stopping at

These are both relatively formal expressions that convey the idea of 'not only...but also'. While they can be used interchangeably in some contexts, に限らず is the more general term simply meaning 'not limited to', while にとどまらず has a nuance of 'going beyond' and can sometimes (not always) place more focus on the expansion, which is typically related to the original limitation in some way.

Both add information beyond an initial point, but the nuance differs. にとどまらず means ‘not limited to’ and emphasizes that the scope goes further than expected, often used in formal or written contexts to highlight expansion, as in 影響は国内にとどまらず海外にも及んだ ('The impact extended beyond the country to overseas.'). だけでなく is more neutral and conversational, simply adding another item without emphasizing scale or extension, as in 英語だけでなく日本語も話せる ('He can speak not only English but also Japanese.').