Class B airspace notation

I thought I understood everything that could be on a sectional, but
before a trip to New York City airspace, I studied the sectional and
terminal area charts a little extra carefully. Just southwest of Newark
(EWR) (and also other places) there is a notation such as 70/+12 in blue
for the floor and ceiling of class B airspace. What does the "+" mean? I
went to the aeronautical chart users guide which has lots of symbols but
not this. They do mention a "-" means up to but not including in one
example not having to do with floor and ceilings, but what could does
this mean for a floor? AGL?