What is an ambident substrate?
There is a subsection in March’s Advanced Organic Chemistry [1, pp. 450–451] that provides a straightforward definition and a couple of examples:

10.G.viii. Ambident Substrates
Some substrates (e.g., 1,3-dichlorobutane) can be attacked at two or more positions, and these may be called ambident substrates.
In the example given, there happen to be two leaving groups in the molecule.
Apart from dichlorobutane, and in general, there are two kinds of substrates that are inherently ambident (unless symmetrical).
One of these, the allylic type, has already been discussed (Sec. 10.E).
The other is the epoxy (or the similar aziridine524 or episulfide) substrate.525
Selectivity for one or the other position is usually called regioselectivity.

Substitution of the free epoxide, which generally occurs under basic or neutral
conditions, usually involves an SN2 mechanism.
Since primary substrates undergo SN2 attack more readily than secondary, unsymmetrical epoxides are attacked in neutral or basic solution at the less highly substituted carbon, and stereospeciﬁcally, with inversion at that carbon.
Under acidic conditions, it is the protonated epoxide that undergoes the
reaction. Under these conditions the mechanism can be either SN1 or SN2.
In SN1 mechanisms, which favor tertiary carbons, attack may be expected be at the more highly substituted carbon, and this is indeed the case.
However, even when protonated epoxides react by what is expected to be an SN2 mechanism, attack is usually at the more highly substituted position.526
This result probably indicates signiﬁcant carbocation character at the carbon (ion pairing, for example).
Thus, it is often possible to change the direction of ring opening by changing the conditions from basic to acidic or vice versa.
In the ring opening of 2,3-epoxy alcohols, the presence of $\ce{Ti(O\textit iPr)4}$ increases both the rate and the regioselectivity, favoring attack at C-3 rather than C-2.527
When an epoxide ring is fused to a cyclohexane ring, SN2 ring opening invariably gives diaxial rather than diequatorial ring opening.528
Cyclic sulfates (108), prepared from 1,2-diols, react in the same manner as epoxides, but usually more rapidly:529


524 Chechik, V.O.; Bobylev, V.A. Acta Chem. Scand. B, 1994, 48, 837.
525 Rao, A.S.; Paknikar, S.K.; Kirtane, J.G. Tetrahedron 1983, 39, 2323; Behrens, C.H.; Sharpless, K.B. Aldrichimica Acta 1983, 16, 67; Enikolopiyan, N.S. Pure Appl. Chem. 1976, 48, 317; Dermer, O.C.; Ham, G.E. Ethylenimine and Other Aziridines, Academic Press, NY, 1969, pp. 206–273.
526 Biggs, J.; Chapman, N.B.; Finch, A.F.; Wray, V. J. Chem. Soc. B 1971, 55.
527 Caron M.; Sharpless, K.B. J. Org. Chem. 1985, 50, 1557. See also, Chong, J.M.; Sharpless, K.B. J. Org. Chem. 1985, 50, 1560; Behrens, C.H.; Sharpless, K.B. J. Org. Chem. 1985, 50, 5696.
528 Murphy, D.K.; Alumbaugh, R.L.; Rickborn, B. J. Am. Chem. Soc. 1969, 91, 2649. For a method of overriding this preference, see McKittrick, B.A.; Ganem, B. J. Org. Chem. 1985, 50, 5897.
529 Gao, Y.; Sharpless, K.B. J. Am. Chem. Soc. 1988, 110, 7538; Kim, B.M.; Sharpless, K.B. Tetrahedron Lett. 1989, 30, 655.

Reference

Smith, M. March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th ed.; Wiley: Hoboken, New Jersey, 2013. ISBN 978-0-470-46259-1.