Within a given area or range; within reach or within the sphere of influence (opposite of 'out of range').
この駅はまだWi‑Fi圏内だが、郊外は圏外になることが多い。
This station is still within Wi‑Fi range, but the suburbs often fall out of range.
彼は首位争いの圏内に入ってきた。
He has entered the range of contention for the top spot.
その国は我が社の影響圏内だ。
That country is within our company's sphere of influence.