On the Profile tab of the Contact Information screen, there is a place to enter what is called a "Key Note". This would be a quick reminder about this customer for you that pops up whenever you select the client.
Also, this Key Note will be shown each time you select a client, but it will not be repeatedly shown for the same client within the "Key Note Supress Time", which is a setting in minutes on the Misc/Setup > Company/Misc Info screen's Order Form Options tab.
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