An example of a participant or subject variable can be

A participant or subject variable is a pre-existing characteristic of the person, such as political affiliation.

Time order is a criterion for causal inference, not a trait of a participant.

Recollection is a cognitive process, not a stable subject characteristic here.

Manipulation refers to what the experimenter does, not to a subject attribute.

Subject variables are selected, not assigned, so the researcher cannot control them.

Age, gender and personality are other common subject variables.