A P11D(b) is a form UK employers use to tell HMRC how much they owe in National Insurance on employee benefits.
If you provide taxable benefits to your employees, such as private medical insurance and company cars, these benefits have to be reported. If not pay rolled the individual details go on P11Ds for each employee, where as the P11D(b) is the summary that combines these details together to confirm the total Class 1A National Insurance you as the employer need to pay on those benefits.
It’s submitted once a year after the end of the tax year (by 6 July), and the Class 1A NIC payment is due shortly after (usually by 19 July by post or 22 July if paying electronically).
Even if you are payrolling your benefits on UK Payroll, you’ll still need to submit a P11D(b) to declare the Class 1A NIC owed. In the software this can be found through the HMRC Documents section > P11Ds and P11D(b)