Players and Members
When a player who is an ICU member plays in an ICU-rated tournament then, unless the tournament controller has done a poor job, when the results are sent to the rating officer the information about that person will include their ICU ID (as well as their name) which serves to uniquely identify them.
On the other hand, for a domestic player who is not yet a member, unless the tournament controller has done an exceptional job, the only information about that person in the rating report will be their name. If the same name were to appear again in a future tournament it would not be possible to confidently assert that it belonged to the same person. In the ICU database at the moment there are over 800 people (just under 7%) who share a name with a least one other person. There are, for example, 8 members called David Murphy.
So, current policy is not to generate new member records (and ICU IDs) for non-members who play in rated tournaments. Without IDs their ratings cannot be tracked from tournament to tournament. In short: a player gets a rating only after they have joined the ICU.
As part of process of joining, players are required to supply their first name, last name, DOB, gender and federation and may also provide a middle name (or initial), email address and club. All of this information helps to accurately identify players both for the rating officer and for tournament controllers.
Retrospective Ratings
Sometimes a member will ask the rating officer if their old games can be rated and supply a list of the old tournaments they played in before joining. While it is technically possible to assign a newly acquired ICU ID to a player in past tournaments and then rerate those tournaments, unfortunately, all tournaments that come after the rerated tournaments would also have to be rerated and if the tournaments were far enough back in time, rating lists would have to be republished as well.
Therefore, the normal policy is not to retrospectively rate old games because of the amount of work involved and given that the problem would not have arisen in the first place had the player joined earlier. In certain circumstances the rating officer may agree, upon request, to undertake such work but is not obliged to. The most favourable circumstances are when the old games occur after the last published rating list.