Not required
Open and not required are semantically related. in possible topic. In some cases you can use "Open" instead an adjective phrase "Not required".
Open
Open adjective - Allowing passage without obstruction.
Not required and open are semantically related. You can use "Not required" instead an adjective "Open".
Mutual synonyms
Both terms in one sentence
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Even though the game is a Wide Open Sandbox, there are only two storylines that you are not required to complete in order to beat the game (three if you count the Shofixti Maidens quest as a separate storyline rather than a part of the Yehat rebellion storyline).
Source: Video Game / Star Control
Cite this Source
Open and Not required. (2016). Retrieved 2026, August 16, from https://thesaurus.plus/related/not_required/open
Not required & Open. N.p., 2016. Web. 16 Aug. 2026. <https://thesaurus.plus/related/not_required/open>.
Open or Not required. 2016. Accessed August 16, 2026. https://thesaurus.plus/related/not_required/open.
Google Ngram Viewer shows how "not required" and "open" have occurred on timeline