There are 22,000 critical workers at home and 2,500 in the office. They're all doing critical work. The difference is that those working on site are not able to work remotely, either because of the equipment or because some of the jobs have to be done on site, like receiving the mail and opening up the paper copies of tax returns, for example.
Anything that has to do with processing the tax returns, the Canada emergency wage subsidy, the CERB, all those programs from the government are critical to the government and to the population. That's what we mean by “critical”, on top of the regular tax work processing the T1s and T2s for corporate and individual tax returns so that people can get their reimbursement on time if they are waiting for one.