Difference between "//" and "/" in XPath?

/ vs // in general

Both child (/) and descendant-or-self (//) are axes in XPath.

• / is short for /child::node()/.

  Use / to select a node's immediate children.

• // is short for /descendant-or-self::node()/.

  Use // to selecta node, its children, its grandchildren, and so onrecursively.

/ vs // with preceding-sibling::*

Your specific question asks about the difference between //preceding-sibling::* and /preceding-sibling::*.

Since your data is offsite and complex, let's consider instead this present and simpler XML:

<r>  <a/>  <b>    <c/>    <d/>  </b></r>

For this XML,

1. /r/preceding-sibling::* selects nothing because r has nopreceding siblings.
2. /r//preceding-sibling::* selects the preceding siblings elements ofall of the descendant or self nodes of r. That is, a, b, c and d.(Remember, /r//preceding-sibling::* is short for /descendant-or-self::node()/preceding-sibling::*, not /descendant-or-self::*/preceding-sibling::*) Note that even though b and d are predecessor siblings to no elements, they are predecessor siblings to text nodes because the above XML has whitespace after b and d. If all whitespace were removed, then only a and c would be selected.
3. /r/descendant::*/preceding-sibling::* selects the preceding sibling elements of all descendant elements of r. That is, a and c. Note that b and d are not selected because they are not preceding sibling elements to any descendant elements of r -- unlike the previous example, text nodes do not qualify.

For your example

//webengagedata/preceding-sibling::* ---> returned 9 results

Because there are only 9 tags which are exact sibling of webengagedatatags thats why it is showing 9 records

//webengagedata//preceding-sibling::* ---> returned 14 results

Here it is considering child tags as well as biziclop said x/descendant-or-self::node()/y

The difference is that x//y is shorthand for x/descendant-or-self::node()/y.

That's all.

So while the first query selects all the descendants of <webengagedata> that have another tag after them, the second only selects the preceding siblings of the tag itself.

The rules of abbreviated xpath syntax are explained here.