Determining the Length and Distance
You may no longer use the DESCRIBE DISTANCE statement to define the lengths and distances of fields. It must be replaced with one of the new statements DESCRIBE DISTANCE ... IN BYTE MODE or DESCRIBE DISTANCE ... IN CHARACTER MODE. 
The DESCRIBE FIELD ... LENGTH statement is also obsolete and must be replaced with one of the new statements DESCRIBE FIELD ... LENGTH ... IN BYTE MODE or DESCRIBE FIELD ... LENGTH ... IN CHARACTER MODE. 
Until now, the DESCRIBE FIELD ... TYPE field statement returned type C for flat structures. In a UP, type u is now returned for flat structures. This can be queried in the ABAP source code. 
There are no changes for the DESCRIBE FIELD ... TYPE ... COMPONENTS ... statement under US. Similarly, the DESCRIBE ... OUTPUT LENGTH ... statement still returns the output length in characters. 

My doubt is for a character type field (i.e. for c,n,d,t or character type structures) if am writing the addition character mode or byte mode its not throwing any error, but for type i, f,x,p or xstring if I give character mode its throwing an error and if I give byte mode its not throwing an error. 

Type i is expected for ilen. 

The variant with the addition IN BYTE MODE determines the length of the data object dobj in bytes. The variant with the addition IN CHARACTER MODE determines the length of the data object dobj in characters. 

When using IN CHARACTER MODE, the data type of dobj must be flat and character-type. You can only specify IN BYTE MODE for deep data types and in this case, the length of the reference (8 bytes) is determined. 

In non-Unicode programs and in releases prior to 6.10, LENGTH can be used without the addition MODE. In this case, the addition IN BYTE MODE is used implicitly 

This covers the all the possiblities of Describe stmt.

Also Use, try this Method:

  REPORT Z_TEST_FIELD_SYMBOLS1.
  TABLES MARA.
  class CL_ABAP_CONTAINER_UTILITIES definition load.

  DATA: I_MARA LIKE MARA OCCURS 0 WITH HEADER LINE.
  DATA: BEGIN OF I_TEST OCCURS 0,
  LINE(8000) TYPE C,
  END OF I_TEST.
  DATA: BEGIN OF I_TEST1 OCCURS 0,
  LINE(1500) TYPE C,
  END OF I_TEST1.
  FIELD-SYMBOLS: <FS> TYPE ANY.
  FIELD-SYMBOLS: <FS2> TYPE ANY.
  FIELD-SYMBOLS: <FS1> TYPE ANY.
  SELECT * UP TO 2 ROWS
  FROM MARA
  INTO MARA.
  call method CL_ABAP_CONTAINER_UTILITIES=>FILL_CONTAINER_C
  exporting IM_VALUE = mara
  importing EX_CONTAINER = i_test-line
  exceptions ILLEGAL_PARAMETER_TYPE = 1
  others = 2.
  APPEND I_TEST.
  ENDSELECT.
  LOOP AT I_TEST.
  WRITE:/ I_TEST.
  ENDLOOP.

  LOOP AT I_TEST.
  call method cl_abap_container_utilities=>read_container_c
  exporting IM_CONTAINER = i_test-line
  importing EX_VALUE = mara
  exceptions ILLEGAL_PARAMETER_TYPE = 1
  others = 2.
  write:/ mara-matnr, mara-mtart.
  clear mara.
  ENDLOOP.

ABAP Tips by :  Satish Chowdary, Nageswar

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