VLOOKUP is your friend
This is a helpful command to use when you have values in different tabs, or even different spreadsheets, on the basic form can be used as follows:
=vlookup(index,range,range-return-column,ordered)
This is a helpful way to append info from another spreadsheet whenever you have at least two columns, the same, in both so you can map one to each other.
Lets say that you have two different spreadsheets that two different teams have been working out, one came from HR and contains contact details for people, on the other hand you have IT, facilities or even Ops filling details for equipment to be delivered.
Two sources of data and one common point, the company's email address, so the following can help you to add for example, the addresses to the list of people and assets.
=vlookup("index",IMPORTRANGE("link_to_the_spreadsheet","mytab!myrange"),"range-return-column","ordered?")