This is a simple vector of line numbers drawn from the state indicator (See The State Indicator).  The most recently activated line is shown first.  If a value corresponds to a defined function in the state indicator, it represents the current line number where the function is either suspended or pendent.

The value of ⎕LC changes immediately upon completion of the most recently activated line, or upon completion of execution within ⍎ or ⎕.  If a ⎕STOP control is set, ⎕LC identifies the line on which the stop control is effected.  In the case where a stop control is set on line 0 of a defined function, the first entry in ⎕LC is 0 when the control is effected.

The value of ⎕LC in a clear workspace is the null vector.

      )SI
#.TASK1[5]*
⍎
#.BEGIN[3]
 
      ⎕LC
5 3

      →⎕LC
      ⎕LC
 
      ⍴⎕LC
0