API 661 Para 7.1.1.2(Air cooler)

Dear All
I had this discussion yesterday about this paragraph from API 661 "The vendor shall make provision for lateral movement of exchanger tube bundles of at least 6Êmm (1/4Êinch) in both directions or at least 12Êmm (1/2Êinch) in only one direction, unless the purchaser and the vendor agree on a greater movement" ,the discussion was about the meaning of the words one directions and both directions , lets say that we have air cooler with tube bundles in the +Z directions , is it true that the meaning of one direction is +X And -X direction,and the meaning of both directions is +X and -X direction ?? or that the both directions is X direction and the Z direction?? .
Thanks

The way I interpret this is thus:

Lateral direction is established by the equipment - let's say the tube bundles are in the Z direction, then lateral is X.

I can either 1) move up to 1/4 inch in positive X and up to 1/4 inch in -X direction, or, 2) move up to 1/2 inch in +X direction and 0 in -X direction , or, 3) move up to 1/2 inch in -X direction and 0 in +X direction.

I could be wrong though.