Beam-end clearances are necessary to to provide adequate assembly and erection tolerances. They take into account rolling mill and fabrication tolerances.
The diagram below indicates the suggested clearance for flexible connections, including:
When there are a string of beams erected along a single line, there is a possibility that an accumulation of tolerances may make the erection somewhat difficult. In such cases it's common practice to make every 4th or 5th beam approximately 5 mm shorter and provide 'Finger Packs' to make up any differences.