You can match employee's Medical FSA contributions from $0 up to the maximum of $3050 (2023 annual limit). Since you can't predict ahead of time what employees elect, if any individual employee sets contribution amounts less than yours, Zenefits simply reduces your contribution to match theirs.
If the FSA is a shortened plan year then the maximum contribution and matching amount will be prorated. So if the company is enrolled in the FSA effective 7/1 through 12 /31, the employee would only be able to contribute half of the maximum amount. This would then be the maximum that the employer is able to match.