Meaning
The kanji 空 on a square button, meaning empty or sky in Japanese. On signs it means vacancy: rooms, seats or spaces still available.
Hotels, homestays and car parks use it to show there is space, and friends use it to say their schedule is open: “Spare room 🈳 this weekend, come stay” Because the character also means sky, some like it for travel themes. Its opposite is 🈵, meaning full.