Can't recognize packets when exchanging data with an electric meter Natris-I100

I am trying to analyse traffic between wireless module RF-433 and electric meter Natris-I100 but some parts of the messages remain unclear.

1. The RF-module sends SNRM frame:
7E A0 21 02 23 21 93 48 55 81 80 14 05 02 07 EE 06 02 07 EE 07 04 00 00 00 01 08 04 00 00 00 01 B5 D4 7E

2. Meter replies by UA frame:
7E A0 21 21 02 23 73 8F 72 81 80 14 05 02 02 00 06 02 02 00 07 04 00 00 00 01 08 04 00 00 00 01 6F EF 7E

3. The RF-module sends data, that I can't understand:
7E A0 1A 02 23 21 10 E6 70 E6 E6 00 FF 01 0A 20 08 CD 2C 39 86 00 21 10 02 3C FA 7E

namely, this part: FF 01 0A 20 08 CD 2C 39 86 00 21 10 02

I assume that the serial number of the counter 021221008639 is being sent.

4. Meter replies: 7E A0 0F 21 02 23 30 39 03 E6 E7 00 FF FD 0F AC 7E

5. RF-module sends data:

7E A0 45 02 23 21 32 68 6D E6 E6 00 FF 01 0A 21 33 00 01 00 10 00 01 00 2B 60 29 A1 09 06 07 60 85 74 05 08 01 01 A6 0A 04 08 00 00 00 00 00 00 98 00 BE 10 04 0E 01 00 00 00 06 5F 1F 04 00 40 1F 7F 00 C8 F8 B9 7E

It seems to be AARQ frame, but some parts of the message remain unclear:

a. FF 01 0A 21
b. 33 00 01 00 10 00 01 00 2B
c. A6 0A 04 08 00 00 00 00 00 00 98 00

6. Reply from meter: 7E A0 0F 21 02 23 52 2D 43 E6 E7 00 FF FD 0F AC 7E

What does the sequence FF FD mean?

Thank you for any help.