My understanding is that it is possible to search the NRS catalogue for records that are held in the Mitchell Library. For example, the below reference for the Calton Old Kirk Sessioni, which the NRS site shows as held off site.
CH2/725 Records of Glasgow, Calton / Calton Old Kirk Session
Some items, yes, but not all. In particular there is no facility to search the Glasgow Poor Law records in the NRS catalogue.
In the context of the NRS catalogue being held 'off site' means that the documents are stored in a different building belonging to the NRS (for example Thomas Thomson House or West Register House) and must be ordered in advance to be produced for readers in General Register House in Edinburgh. It does not mean that the item referred to is held in a different archive elsewhere.
Initially almost all the Kirk Session records were directly held by the NRS. Some of them, evidently not including the Calton Old Kirk Session records, have been returned to local archives.
This is an extract from the Glasgow City Archives web site: "
Poor law archives are our most important source for the family historian. These are the records of the poor law authorities, which contain over one million applications for poor relief. We hold records for the following areas: • Glasgow, 1851-1948 • Barony, 1861-1898 (part of Glasgow from 1898) • Govan, 1876 – 1930 (part of Glasgow from 1930) • Bute, West Dunbartonshire, South Lanarkshire and Renfrewshire (not including Paisley), often dating from 1845. Poor relief applications are particularly useful for researching people who migrated to the city, including those from Ireland. Records are generally closed for 100 years for children and 75 years for adults. Poor law authorities were responsible for children separated from their parents and the records can contain information about children who were fostered or adopted. Access restrictions apply to fostering or adoption records that are less than 100 years old.".
There is also a guide to the Poor Law holdings
https://www.glasgowlife.org.uk/libraries/family-history/research-guides/poor-law-from-1845 and a leaflet specifically about the Glasgow Archives' Poor Law material
https://www.glasgowlife.org.uk/media/cbndxgh0/archives-poor-law-leaflet-v3.pdf