From the pattern parenting plan: "The children named in this parenting plan are scheduled to reside the majority of the time with the [ ] petitioner [ ] respondent. This parent is designated the custodian of the child(ren) solely for purposes of all other state and federal statutes which require a designation or determination of custody. This designation shall not affect either parent’s rights and responsibilities under this parenting plan." The pattern forms are free at http://www.courts.wa.gov/forms/ .
Being the "custodial parent" simply means the child spends more than with that parent than the other parent. While it may theoretically be possible for a child to spend exactly 50% of the child's time with each parent, the child likely does spend more than 50% of the child's time with one parent when the hours are accurately counted.
That one parent is the custodial parent does not necessarily mean that the other parent has fewer rights or responsibilities to the child. It just means one parent has more time allocated in the parenting plan to be with the child.
You should review your specific facts with your attorney so that you can receive informed advice applicable to your specific situation.