In order to develop F1 seeds the male sterile line should be pollinated with the male fertile line. Maintainer line has the same nuclear genes as the sterile one but differs from it in the composition of mitochondrial (in most cases) genetic factors that cause male sterility in plants (by other words it has "fertile" cytoplasm).

In contrast to restorer line the maintainer line lacks nuclear genes that supress male sterile phenotype therefore when you cross male sterile line with its maintainer you will get the same male sterile genotype for next crossing.

Breeding mainainer line is a long term process. If you want to develop maintainer for a sterile line you need carry 5-6 backcrosses between sterile line and its future fertile analog in oder to transfer nuclear genes of sterile line into the fertile cytoplasm.

Thank you so much Irina, Can you suggest a book with Diagrammatic presentation of CGMS & other types of sterility

Manual of hybrid Rice by SS virmani

It appears that in some cases nature can render male sterile genotypes pollen fertile for limited periods,  although the male sterile plant lacks restorer alleles in its nuclear genome and returns to producing aborted stamens after an interruption. I observed such a case in an East Mediterranean population of Anemone coronaria L., that was monitored for over 30 years at the end of the last century (Horovitz, 1984;  Horovitz in preparation). During that  time the population underwent a shift from its original site that became a temporary quarry. Yet nuclear-cytoplasmic male sterility remained constant at a rate of about 5%.

In its natural populations in that area, the first flowers in the season only develop after five to eight rosette leaves have been formed. This  juvenile period of the seedling or flowerless period of an adult corm can extend over several years,  and growth is interrupted by summer dormancies during  the rainless hot months of May to September (Kadman-Zahavi et al. 1984. First seasonal flowers that must have reached advanced stages of development before the interruption  are fully pollen fertile. Some of the subsequent flowers are partially pollen fertile. Together, about 25% of the flowers produced  in the male sterile genotype produce pollen. The underlying factors that silence or override recessive alleles in the nuclear genome and/or alter the functioning of mitochondrial factors have not been studied. Perhaps one may speculate that, as foliage leaf development at the periphery  of a floral meristem is arrested by dormancy, a temporary surplus of resources is created that  are diverted to continuations of stamen development  

The flowers are protogynous with a one-to three-day female period during which they can be pollinated very soon after flower opening. This is followed by an interval of one or more weeks before the first anthers dehisce. There is a linkage between male sterility and early flowering. As a result, genetically male sterile plants interbreed  and mate assortatively  during their early season male fertile period. In the context of the question asked here, seed from their earliest flowers in the season  can theoretically constitute female maintainer lines for F1 hybrid  breeding in A. coronaria .

The phenomenon observed affords a means of tracing irregular floral development and can be of wider interest to studies of gender variation (Meagher, 2007).

Another point that has to be made is that early pollen, carrying male sterility alleles will also reach some first flowers of  bisexual plants that will otherwise remain unfertilized because of protogyny, In addition to assortative mating among male steriles, this spread of alleles can explain the rare long-time  persistence of the unisexual genotype.

References:

Horovitz, A. 1984. Seasonal   pollen fertility in male sterile Anemone coronaria. Abs. Ann.  Meet. Bot. Soc. Amer., Amer. J. Bot. Amer. 71 Suppl.:98-99.

Horovitz, A. In preparation. Lessons to be learned from the reproductive biology of East Mediterranean  Anemone  coronaria.

Kadman-Zahavi, A., Horovutz, A. and Y. Ozeri, 1984. Long-day induced dormancy in Anemone coronaria.  Ann. Bot. 53: 213-217.

Meagher T. R. 2007. Linking the evolution of gender variation to floral development.  Ann. Bot.100: 165-176

Do you really want to know the answer. I guess we all know the answer.