The House of Councillors explains on its website that the laws and rules for the Diet cannot cover every situation. Where they say nothing, the house follows senrei, precedent. Precedents fill the gaps in the written rules so that proceedings can run smoothly.
The House of Councillors collects them in the Sangiin senreiroku, a record of precedents in 32 chapters. The House of Representatives keeps its own collection, the Shūgiin senreishū. The National Diet Library advises that the written rules must be read together with these collections.