Unassigned seat

Hi, I have trouble understanding the guidelines. The flix website says:

"For tickets with no assigned seat

• You are free to choose an unassigned seat, which are marked with a green sticker.
• It is recommended to board the bus early to find the seat you want.
• Passengers with limited mobility are given priority for seating."

For my upcoming trip, I have an unassigned seat. Does it mean my seat is still 100% guaranteed, but its position is not sure, or is there a chance I might not get on the bus if the green seats are all occupied? I want to know if me buying the ticket means there will always 100% be a free seat for me available. Thanks for your help yall!