ts_rank error/result of '1e-020'

I have a workaround to the error/result, but am wondering what the result of
ts_rank of '1e-020' represents?

Here is the original:

select
ts_rank(to_tsvector('a_a_do_ug_read_retreqmon_ptam'),to_tsquery('a_a_do_ug_read_retrmso.com_ptam'))