I have programmed:
If numberfield = "" Then numberfield = @number
on Entry
and
I have typed the number 159 in the Application Property Manager.
This works well and the next record that I want to add will show 160 in the numberfield and 161 for the next etc...
However, if, after I enter the Add Mode and the correct number appears, I decide to Exit the Add mode and not add any records, the next time I try to add a record it will give me the number after the one that appeared before and I wind up loosing a number.
I would like to be able to use the number that appeared the first time without having to go to Design Mode and change the initial @number value.

Why is it better to use:
If(@IsNew) and If(@Modified) instead of:
If numberfield = ""    and  @number ?

Thanks for the very clear explanation.
Now, what about the use of @number versus @XLookupR or Global value ?

Thanks again.

I like the @XLR idea BUT what if I want to generate the next available number and don't really have a "key" field to latch on to?

Example: Number field needs to be incremented by last highest number used plus one. Suppose I want the next number generated as soon as I enter the database. All fields are blank. There are no blank fields in any of my existing (saved) records. (I used "blank" as an example because ANY of my records could contain any of a couple of dozen different values, and I don't want to only add one number based on the last matching value of a particular field-- I merely want to keep a record number associated with a record, which makes searching very easy.)

I tried the following using Number as the field I want to increment:

Number = @XLR(@Fn, Number, "Number", "Number") +1

This, of course, always returns "1" as my new record number.

So - how do I add "1" to the last record number generated when first entering a blank record?

Change From:
Number = @XLR(@Fn, Number, "Number", "Number") +1

Change To:
Number = @ToNumber( @XLR(@Fn, 99999999, "Number", "Number") ) +1

1.  The @XLR returns a string so you need to convert the XLR result to a number:
@ToNumber( @XLR(........) ) +1 

2.  The second value of that expression is the number the XLR is looking for.  When it does not find that value, it keeps decreasing by one, until it finds the highest value in "Number".  And then it does +1 to increase the value by one.

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Four examples:
1.  If records have "Number" values from 1 to 158: the XLR looks for 99999999, does not find it, but does get the highest value,158, adds one, and returns a value of 159 for the next value to be assigned.

2. If entering the first record: it does not find 99999999, does  get the value of 0, adds one, and returns a value of 1 of the next value to be assigned.

3a.  If you had records with "Number" values from 1 to 278, and used XLR starting with 150: then the XLR would return the value of 151 because 150 does exist, and one is added. 

3b.  Same as above, but if you used XLR  starting with 999, then the XLR would return 279, because 999 did not exist, the closest value to it was 278, one was added and 279 was returned.
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The initial 99999999 is just a very high number that will always be higher than any value assigned to "Number" in normal operations.