National Structural Concrete Specification for Building Construction; 4th edition, complying with BS EN 13670:2009
"Lift shafts require a minimum plumb dimension and a positive only tolerance".
https://engineeringsurveyor.com/software/NSCS-Edition-4.pdf
Unable to copy & paste from this, but see §10, "Geometric Tolerances", and especially §10.2.1 on "Inclination".
By my reading this is about deviation of the shaft from perfectly vertical. Hence the reference to a plumb, as in plumb line.
Cf. https://en.wikipedia.org/wiki/Plumb_bob

The lift will presumably want to hang vertically. If the shaft has the necessary width to accommodate the lift but has been built on a slight angle, then the lift may knock into the walls of the shaft — especially in a tall building with a long shaft, where small angular deviations would accumulate into large horizontal 'errors'.

As it is only possible for the shaft to be less vertical than the plumb line — it cannot be more vertical than the plumb line — only a positive tolerance should be specified (to be added onto the minimum [breadth] dimension).

Constructing the shaft wider than this minimum value will allow it to accommodate a lift safely even if there are some deviations from true vertical.
The allowance to be made (the magnitude of the tolerance) may vary from one building to another.

I do not claim the above phrasing to be perfectly idiomatic German, but I think here the main issue is understanding the meaning.

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Note added at 17 hrs (2019-08-15 15:48:15 GMT)
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By the way, "inclination" in the cited text refers to the 'angle' or 'lean' or 'slope'.
http://www.mathwords.com/a/angle_inclination.htm

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Note added at 17 hrs (2019-08-15 15:56:17 GMT)
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"Builder's work — lift well:
· Calculations with regard to the architect's plans and structural loadings.
· Form a plumb lift well and pit according to the architect's drawings and to tolerances acceptable to the lift manufacturer (known as Nominal Minimum Plumb — the basic figures in which the lift equipment can be accommodated)."
https://epdf.pub/queue/building-services-handbook.html

"Form a plumb lift well and pit" is indicating a vertical construction. Although you'd think that'd 'go without saying' ....
"Nominal Minimum Plumb — the basic figures in which the lift equipment can be accommodated)." I believe "ideal" or "bare" (as in "bare minimum") would be better words than "basic" here [I am ignoring "Nominal"].

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Note added at 17 hrs (2019-08-15 16:07:00 GMT)
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BTW, very tall buildings ('skyscrapers') sway with the wind: even if they could be built to be perfectly vertical in the absence of wind, they would deviate from vertical in the presence of wind.

"A 40-storey building may sway a foot to the left, a foot to the right. [....] A 100-storey building, by comparison, may move on the order of two-and-a-half to three feet to each side, [...]."
https://www.gizmodo.com.au/2015/06/how-much-do-skyscrapers-a...

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Note added at 17 hrs (2019-08-15 16:20:16 GMT)
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I think this interpretation is compatible with the "lichte Abmessungen" description uncovered by Rolf Keller, except that "minimum plumb" seems to explicitly also convey the key assumption behind determination of the minimum.

licht: .../"[...]; spärlich"/"(von Öffnungen o. Ä.) von der einen zur anderen inneren Begrenzungsfläche gemessen"
https://www.duden.de/rechtschreibung/licht