Topical Theme

Here is a sentence from the Wikipedia page for Japanese writing:

The modern Japanese writing system uses a combination of logographic kanji and syllabic kana

The part highlighted in bold is called the ideational, or topical, theme. These show the main topic, or what the sentence is about. Sometimes, sentences do not begin with the grammatical subject of the sentence. The topical theme can also be a prepositional phrase:

In modern Japanese, the hiragana and katakana syllabaries each contain 46 basic characters;

an adverb of time:

Even today Japanese high schools teach kanbun as part of the curriculum;

or subordinate clause:

when used as a suffix meaning “try out”, the whole verb is typically written in hiragana.

All clauses, however, must have at least one topical theme.