Symbol is written as "Symbol "◎◎" [Text]" in the card text, where "◎◎" portion could be any words. If your Z/X with "◎◎" in its card name is currently on Square or in Charge, the ability written on the [Text] portion will become available.
The current reminder text of Session ability is shown below:
Symbol "◎◎" (Available if your Z/X with "◎◎" in its card name is on Square or in Charge.)
1119.1 Symbol ability is an ability with continuing effect that becomes available if the card with the ability meet a specific requirement.
1119.1a Parameter obtained by the ability itself cannot be referred to meet the requirement for Symbol ability.
1119.1b If there are 2 or more related interdependent effects to meet the requirement for Symbol ability, they are applied in order starting from the earliest reference time point (811.3a).
1119.2 Symbol ability is written in the form of "Symbol "◎◎" [Text] ". This ability becomes available if there is your Z/X with "◎◎" in its card name on Square or in Charge.
1119.3 Symbol ability is available (803) regardless of card type and which zone it's currently present. However, the ability shown in the [Text] of Symbol ability only becomes available (803.1) if it met its specific requirements.