Hi Sue,
First, you should always consider the potential use of Database Logging as a means of tracking the insert, delete, and update of any record type in the system. For example, once activated for the Trade agreement table (PriceDiscTable), any change to a specific record in that table will leave an excellent audit trail, showing each individual field that was changed (before and after values), and who made the change. Of course you can always activate the "last modified when and by" fields on any table, but Database Logging obviously shows a more complete trail than just the last operation. Since they work system-wide, they are features to consider and understand as general tools, to be tempered with their potential performance impact on highly active tables (InventTrans, for example, would not be a good idea).
However, in the specific case of Trade agreements, you can use the built-in effective/expiration date nature of the records themselves to accomplish a similar form of audit trail, and in a more useful way. For example, when you pull a set of existing records into a Price/discount journal to post an update, you can expire the original records as of a given date, and then copy those records into the journal, make those newly copied records effective the day after the other ones expire, and then alter the prices as needed (or even in one operation, as in add 5%). Then posting the price/discount journal leaves the original records in place now expired and creates the new records, all in one action. Some companies simply re-use the same records over and over, but using trade agreements in the way I have described offers some advantages. First, the audit trail is automatic and complete (as long as you never delete records except to eliminate true mistakes), and second you can post changes to become effective at a future date without any future effort. When entering orders (both sales and purchase), the date which is used to find effective trade agreements can either be the order date or the delivery date, as controlled by sales and purchase parameters respectively.
In terms of inquiry, from the released product you can go directly to sales and purchase trade agreements, and see the entire collection of records which form the audit trail. I would recommend not trusting or even using the last purchase price field on the released product.
You can, of course, be as descriptive or brief with your journal descriptions as you like. They either will or won't help you to locate a journal you might be interested in later depending. I usually recommend once a journal is posted that any physical paperwork (price increase notice or schedule from the vendor) be scanned and attached directly to the journal header using Document handling. There's simply no better way to justify what and why a journal is posted than to see the original piece of paper that was in hand at the time you did it.
Good luck. Hope this helps!