"boot" for bootstrapped CI, "Wald" for estimated. Only in the case of a mixed-effect model. See confint.merMod.
nsim
how many times should it bootstrap the confindence intervals (only if method = "boot").
add.stars
Add significance stars.
ddf
Denominator degrees of freedom approximation (only if fit is an instance of lmerTest::merModLmerTest, ignored otherwise). If `NULL`, then the normal approximation (treat t-values as z-values) is used for lmer objects fit with lme4 and the default Satterthwaite is used for models fit with lmerTest.