In the House of Commons, Grand Committees are a forum for all the MPs who represent a particular nation within the UK to meet together and consider matters that relate only to Wales or only to Scotland or Northern Ireland. Since 2015, Legislative Grand Committees for English or English and Welsh only MPs have also been used under the 'English votes for English laws' procedures.
In the House of Lords, debates on statutory instrument (SIs) can be held in either the Chamber or in Grand Committee. Decisions can be made in Grand Committee but divisions (votes) only take place in the Chamber. If the SI is debated (considered) in Grand Committee, it must be approved by a separate motion in the Lords Chamber at a later date (although this is usually done without further comment). If the debate takes place in the Chamber both discussion and approval can happen at the same time. All members of the Lords can attend and speak in Grand Committee, it refers to the location rather than a specific committee with a limited membership.