This seems like an odd design though that may be the abstract labeling you chose to use to describe the problem.
On what criteria would you base your search of the "other" table? You wouldn't use the the data in field a to find a record in other with the same value in z as then you would be copying the same value in z that is already present in field a back to field a--which makes no sense.
Reposting your example with more detail and real field, table names will make it easier to understand your question.
layout "work" fields "AOtherCode" "BOtherCode" "COtherCode" "DOtherCode" "EOtherCode" "FOtherCode" relationship layout "other" field "OtherCode"
for instance i'm in layout "work" and have a button next to "AOtherCode" that opens file layout "other" i then search for code that means "Bank Overdraft Fee" which is number 24. after finding this code want to go back to "work" and have 24 inserted automatically into "AOtherCode .
This can be done with a two column value list that lists values from the other field. Make column 1 your code (24) and column 2 the name (Bank Overdraft Fee).
Then, you can format your "work" fields with this value list.
currently have 450 records in "other" layout which could change any day. do you still think you value list would work?
Yes it can work. The frequent changes will be automatically reflected in your drop down if you set the value list to draw it's values from the table. The number of items in your drop down (400+) is a bit unweildy. There may be ways to make that list easier to work with by making it a conditional value list or a script that finds the data for you may prove to be the best. I can't tell from here as I have a very incomplete picture of your database.
Here's a thread on setting up conditional value lists:
Custom Value List?
If you want to try a scripted approach that looks up a matching value, the tutorial that this link connects you to might be adapted to serve. In the tutorial, the script looks up a matching serial number. In your case, you'd adapt the script to look up your code instead of the serial number.