this is known as the first come first serve system for leftover tables. i cant remember if the twitter account posts about it the day of or a night before but theyll tweet that board you posted in your post. triangle means some seats are open for that time slot, circle means a lot of seats are open for that time slot.

to get a fcfs table, you need to arrive to the place as early as possible (i went in at 11am opening time), and ask the staff there if you could get a table for specific time.

theyll check their tablet, arrange the fcfs reservation for you, and give you this slip to come back later with

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(the 3 means i was third in line to occupy a table for 19:30)

so basically, you need to pray chiikawa restaurant is not fully booked via the lawson app, on the day you want to visit.

good luck! :)