In the case of perishable goods IE sausages then provided you keep them in the fridge they will be good until there use by date, however if you open a jar or tin then its use by date is significantly reduced
On the packaging there will normally be some information on storage, eg once opened keep refrigerated and use within 3 days. If not then I would think that it's OK to keep them until the use by date.
Some food is packaged in a protective atmosphere and the packaging is airtight. These should NOT be kept more than 24 hours, preferably less, after opening regardless of the use by date.
If the packaging is not airtight then they should be OK a bit longer.