When looking for an ancestor who served with the Canadian Expeditionary Force the very first place you can go are the Attestation Papers at Library and Archives Canada data base “Soldiers of the First World War“. Service numbers are the primary identifier in the Canadian Corps.
When the Battalions were been organized service numbers were allotted in number blocks. This did not last so do not assume that because your veteran has a service number in the 6000 block he served in the 1st.Battalion. Probably because of the tremendous growth in numbers in the Canadian Corps there was a certain amount of confusion in assigning numbers. There are some duplications in the early years. By 1915 this system was re-organized (of course with some exceptions), and each unit as it was organized was given a block of numbers and each soldier served throughout the remainder of the war with an unique service number. Even so, you do need that soldier’s service file if want to identify exactly which Battalion he served in in the trenches. The Battalion that he joined in Canada was not necessarily the Battalion he was sent to as a reinforcement once he reached France.
Following British army traditions, officers were not assigned service numbers. However, with the high casualty rate for officers in the trenches NCO’s were promoted. After promotion they were identified by name not service number from the date of their promotion. For these veterans when requesting service files from Library and Archives Canada the researcher should use both the service number and name.