  We study gapped boundaries of Abelian type-I fracton systems in three spatial
dimensions. Using the X-cube model as our motivating example, we give a
conjecture, with partial proof, of the conditions for a boundary to be gapped.
In order to state our conjecture, we use a precise definition of fracton
braiding and show that bulk braiding of fractons has several features that make
it \textit{insufficient} to classify gapped boundaries. Most notable among
these is that bulk braiding is sensitive to geometry and is "nonreciprocal,"
that is, braiding an excitation $a$ around $b$ need not yield the same phase as
braiding $b$ around $a$. Instead, we define fractonic "boundary braiding,"
which resolves these difficulties in the presence of a boundary. We then
conjecture that a boundary of an Abelian fracton system is gapped if and only
if a "boundary Lagrangian subgroup" of excitations is condensed at the
boundary, this is a generalization of the condition for a gapped boundary in
two spatial dimensions, but it relies on boundary braiding instead of bulk
braiding. We also discuss the distinctness of gapped boundaries and transitions
between different topological orders on gapped boundaries.
