"is registration-pending FOO7BAR confusingly similar to the registered trademark FOOBAR, even if they don't belong to the same class?"
There are 9 factors the PTO uses to answer that question, so you'll need to provide more context. The factors come from In re E. I. du Pont de Nemours & Co., 476 F.2d 1357, 177 USPQ 563 (C.C.P.A. 1973).
The similarity or dissimilarity of the marks in their entireties as to appearance, sound, connotation and commercial impression.
The similarity or dissimilarity and nature of the goods or services as described in an application or registration or in connection with which a prior mark is in use.
The similarity or dissimilarity of established, likely-to-continue trade channels.
The conditions under which and buyers to whom sales are made, i.e. 'impulse' vs. careful, sophisticated purchasing.
The fame of the prior mark (sales, advertising, length of use).
The number and nature of similar marks in use on similar goods.
The nature and extent of any actual confusion.
The length of time during and the conditions under which there has been concurrent use without evidence of actual confusion.
The variety of goods on which a mark is or is not used (house mark, 'family' mark, product mark).
